Make it so that most options in the config are optional, and commented out in
the generated config.
The reasons this is a good thing are as follows:
* If we decide that we should change the default for an option, we can do so,
and only those admins that have deliberately chosen to override that option
will be stuck on the old setting.
* It moves us towards a point where we can get rid of the super-surprising
feature of synapse where the default settings for the config come from the
generated yaml.
* It makes setting up a test config for unit testing an order of magnitude
easier (see forthcoming PR).
* It makes the generated config more consistent, and hopefully easier for users
to understand.
The general idea here is that config examples should just have a hash and no
extraneous whitespace, both to make it easier for people who don't understand
yaml, and to make the examples stand out from the comments.
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.
