* Expose `return_html_error`, and allow it to take a Jinja2 template instead of a raw string
* Clean up exception handling in SAML2ResponseResource
* use the existing code in `return_html_error` instead of re-implementing it
(giving it a jinja2 template rather than inventing a new form of template)
* do the exception-catching in the REST layer rather than in the handler
layer, to make sure we catch all exceptions.
By persisting the user interactive authentication sessions to the database, this fixes
situations where a user hits different works throughout their auth session and also
allows sessions to persist through restarts of Synapse.
If an error happened while processing a SAML AuthN response, or a client
ends up doing a `GET` request to `/authn_response`, then render a
customisable error page rather than a confusing error.
Turns out that figuring out a remote user id for the SAML user isn't quite as obvious as it seems. Factor it out to the SamlMappingProvider so that it's easy to control.
We want to assign unique mxids to saml users based on an incrementing
suffix. For that to work, we need to record the allocated mxid in a separate
table.
