* 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.
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.
This implements both a SAML2 metadata endpoint (at
`/_matrix/saml2/metadata.xml`), and a SAML2 response receiver (at
`/_matrix/saml2/authn_response`). If the SAML2 response matches what's been
configured, we complete the SSO login flow by redirecting to the client url
(aka `RelayState` in SAML2 jargon) with a login token.
What we don't yet have is anything to build a SAML2 request and redirect the
user to the identity provider. That is left as an exercise for the reader.
