Implement a username picker for synapse (#8942)

The final part (for now) of my work to implement a username picker in synapse itself. The idea is that we allow
`UsernameMappingProvider`s to return `localpart=None`, in which case, rather than redirecting the browser
back to the client, we redirect to a username-picker resource, which allows the user to enter a username.
We *then* complete the SSO flow (including doing the client permission checks).

The static resources for the username picker itself (in 
https://github.com/matrix-org/synapse/tree/rav/username_picker/synapse/res/username_picker)
are essentially lifted wholesale from
https://github.com/matrix-org/matrix-synapse-saml-mozilla/tree/master/matrix_synapse_saml_mozilla/res. 
As the comment says, we might want to think about making them customisable, but that can be a follow-up. 

Fixes #8876.

1 files changed, 27 insertions, 0 deletions

diff --git a/synapse/res/username_picker/style.css b/synapse/res/username_picker/style.css
new file mode 100644
index 0000000000..745bd4c684
--- /dev/null
+++ b/synapse/res/username_picker/style.css
@@ -0,0 +1,27 @@
+input[type="text"] {
+  font-size: 100%;
+  background-color: #ededf0;
+  border: 1px solid #fff;
+  border-radius: .2em;
+  padding: .5em .9em;
+  display: block;
+  width: 26em;
+}
+
+.button--disabled {
+  border-color: #fff;
+  background-color: transparent;
+  color: #000;
+  text-transform: none;
+}
+
+.hidden {
+  display: none;
+}
+
+.tooltip {
+  background-color: #f9f9fa;
+  padding: 1em;
+  margin: 1em 0;
+}
+