Add experimental "dont_push" push action to suppress push for notifications

This is a potential solution to https://github.com/vector-im/riot-web/issues/3374
and https://github.com/vector-im/riot-web/issues/5953
as raised by Mozilla at https://github.com/vector-im/riot-web/issues/10868.

This lets you define a push rule action which increases the badge count (unread notification)
count on a given room, but doesn't actually send a push for that notification via email or HTTP.
We might want to define this as the default behaviour for group chats in future
to solve https://github.com/vector-im/riot-web/issues/3268 at last.

This is implemented as a string action rather than a tweak because:
 * Other pushers don't care about the tweak, given they won't ever get pushed
 * The DB can store the tweak more efficiently using the existing `notify` table.
 * It avoids breaking the default_notif/highlight_action optimisations.

Clients which generate their own notifs (e.g. desktop notifs from Riot/Web
would need to be aware of the new push action) to uphold it.

An alternative way to do this would be to maintain a `msg_count` alongside
`highlight_count` and `notification_count` in `unread_notifications` in sync responses.
However, doing this by counting the rows in `events` since the `stream_position`
of the user's last read receipt turns out to be painfully slow (~200ms), perhaps
due to the size of the events table.  So instead, we use the highly optimised
existing event_push_actions (and event_push_actions_staging) table to maintain
the counts - using the code paths which already exist for tracking unread
notification counts efficiently.  These queries are typically ~3ms or so.

The biggest issues I see here are:
 * We're slightly repurposing the `notif` field on `event_push_actions` to
   track whether a given action actually sent a `push` or not.  This doesn't
   seem unreasonable, but it's slightly naughty given that previously the
   field explicitly tracked whether `notify` was true for the action (and
   as a result, it was uselessly always set to 1 in the DB).
 * We're going to put more load on the `event_push_actions` table for all the
   random group chats which people had previously muted. In practice i don't
   think there are many of these though.
 * There isn't an MSC for this yet (although this comment could become one).

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