Allow for the creation of a support user.
A support user can access the server, join rooms, interact with other users, but does not appear in the user directory nor does it contribute to monthly active user limits.
This is largely a precursor for the removal of the bundled webclient. The idea
is to present a page at / which reassures people that something is working, and
to give them some links for next steps.
The welcome page lives at `/_matrix/static/`, so is enabled alongside the other
`static` resources (which, in practice, means the client API is enabled). We'll
redirect to it from `/` if we have nothing better to display there.
It would be nice to have a way to disable it (in the same way that you might
disable the nginx welcome page), but I can't really think of a good way to do
that without a load of ickiness.
It's based on the work done by @krombel for #2601.
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.
This is mostly factoring out the post-CAS-login code to somewhere we can reuse
it for other SSO flows, but it also fixes the userid mapping while we're at it.
* Rip out half-implemented m.login.saml2 support
This was implemented in an odd way that left most of the work to the client, in
a way that I really didn't understand. It's going to be a pain to maintain, so
let's start by ripping it out.
* drop undocumented dependency on dateutil
It turns out we were relying on dateutil being pulled in transitively by
pysaml2. There's no need for that bloat.
* Clean up the CSS for the fallback login form
I was finding this hard to work with, so simplify a bunch of things. Each
flow is now a form inside a div of class login_flow.
The login_flow class now has a fixed width, as that looks much better than each
flow having a differnt width.
* Support m.login.sso
MSC1721 renames m.login.cas to m.login.sso. This implements the change
(retaining support for m.login.cas for older clients).
* changelog
