Use attrs internally for the URL preview code & add documentation. (#10753)

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-URL Previews
-============
-
-Design notes on a URL previewing service for Matrix:
-
-Options are:
-
- 1. Have an AS which listens for URLs, downloads them, and inserts an event that describes their metadata.
-   * Pros:
-     * Decouples the implementation entirely from Synapse.
-     * Uses existing Matrix events & content repo to store the metadata.
-   * Cons:
-     * Which AS should provide this service for a room, and why should you trust it?
-     * Doesn't work well with E2E; you'd have to cut the AS into every room
-     * the AS would end up subscribing to every room anyway.
-
- 2. Have a generic preview API (nothing to do with Matrix) that provides a previewing service:
-   * Pros:
-     * Simple and flexible; can be used by any clients at any point
-   * Cons:
-     * If each HS provides one of these independently, all the HSes in a room may needlessly DoS the target URI
-     * We need somewhere to store the URL metadata rather than just using Matrix itself
-     * We can't piggyback on matrix to distribute the metadata between HSes.
-
- 3. Make the synapse of the sending user responsible for spidering the URL and inserting an event asynchronously which describes the metadata.
-   * Pros:
-     * Works transparently for all clients
-     * Piggy-backs nicely on using Matrix for distributing the metadata.
-     * No confusion as to which AS
-   * Cons:
-     * Doesn't work with E2E
-     * We might want to decouple the implementation of the spider from the HS, given spider behaviour can be quite complicated and evolve much more rapidly than the HS.  It's more like a bot than a core part of the server.
-
- 4. Make the sending client use the preview API and insert the event itself when successful.
-   * Pros:
-      * Works well with E2E
-      * No custom server functionality
-      * Lets the client customise the preview that they send (like on FB)
-   * Cons:
-      * Entirely specific to the sending client, whereas it'd be nice if /any/ URL was correctly previewed if clients support it.
-
- 5. Have the option of specifying a shared (centralised) previewing service used by a room, to avoid all the different HSes in the room DoSing the target.
-
-Best solution is probably a combination of both 2 and 4.
- * Sending clients do their best to create and send a preview at the point of sending the message, perhaps delaying the message until the preview is computed?  (This also lets the user validate the preview before sending)
- * Receiving clients have the option of going and creating their own preview if one doesn't arrive soon enough (or if the original sender didn't create one)
-
-This is a bit magical though in that the preview could come from two entirely different sources - the sending HS or your local one.  However, this can always be exposed to users: "Generate your own URL previews if none are available?"
-
-This is tantamount also to senders calculating their own thumbnails for sending in advance of the main content - we are trusting the sender not to lie about the content in the thumbnail.  Whereas currently thumbnails are calculated by the receiving homeserver to avoid this attack.
-
-However, this kind of phishing attack does exist whether we let senders pick their thumbnails or not, in that a malicious sender can send normal text messages around the attachment claiming it to be legitimate.  We could rely on (future) reputation/abuse management to punish users who phish (be it with bogus metadata or bogus descriptions).   Bogus metadata is particularly bad though, especially if it's avoidable.
-
-As a first cut, let's do #2 and have the receiver hit the API to calculate its own previews (as it does currently for image thumbnails).  We can then extend/optimise this to option 4 as a special extra if needed.
-
-API
----
-
-```
-GET /_matrix/media/r0/preview_url?url=http://wherever.com
-200 OK
-{
-    "og:type"        : "article"
-    "og:url"         : "https://twitter.com/matrixdotorg/status/684074366691356672"
-    "og:title"       : "Matrix on Twitter"
-    "og:image"       : "https://pbs.twimg.com/profile_images/500400952029888512/yI0qtFi7_400x400.png"
-    "og:description" : "“Synapse 0.12 is out! Lots of polishing, performance & bugfixes: /sync API, /r0 prefix, fulltext search, 3PID invites https://t.co/5alhXLLEGP”"
-    "og:site_name"   : "Twitter"
-}
-```
-
-* Downloads the URL
-  * If HTML, just stores it in RAM and parses it for OG meta tags
-    * Download any media OG meta tags to the media repo, and refer to them in the OG via mxc:// URIs.
-  * If a media filetype we know we can thumbnail: store it on disk, and hand it to the thumbnailer. Generate OG meta tags from the thumbnailer contents.
-  * Otherwise, don't bother downloading further.