Extends #5794 etc to the SimpleHttpClient so that it also applies to non-federation requests.

Fixes #7092.

Fixes #7043

This reverts commit 54dd2862, reversing
changes made to 6640460d.

Fixes #7065 

This is basically the same as https://github.com/matrix-org/synapse/pull/6847 except it tries to populate events from `state_events` rather than `current_state_events`, since the latter might have been cleared from the state of some rooms too early, leaving them with a `NULL` room version.

Move the default SAML2 error HTML to a dedicated file

Skip the correct visibility checks when checking the visibility of the state at a given event

Also add some JS to it to process any error we might have in the URI
(see #6893).

Co-Authored-By: Brendan Abolivier <github@brendanabolivier.com>

SAML2: render a comprehensible error page if something goes wrong

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.