More Apache ProxyPass directives need 'nocanon'

Invitations weren't working for me until I added 'nocanon' to these additional places. Until then, invitations failed with "Invalid signature for server ..." errors, as in https://github.com/matrix-org/synapse/issues/3294 .

I didn't check whether the user_directory/search proxy line also needs it, I just assumed it should have it too.

The other two proxy lines in this example also include a 'retry=0' parameter. That's a separate issue; I haven't touched it here.

Fix force-pulling of mautrix-whatsapp

Add haproxy doc

Fixes #70 (Github Issue).

Fixes #71 (Github Issue).

Add Slack bridging support

Add mautrix-hangouts bridge role and documentation

Describe setting up SMTP server with Sendgrid

Update synapse (1.3.0 -> 1.3.1) and riot-web (1.3.2 -> 1.3.3)

Thanks to @ralfi!

Contributed by @ralfi.

missing Map /.well-known/matrix/server for server discovery

Continuation of #246 (Github Pull Request)

Update synapse (1.2.1 -> 1.3.0)

Discord client IDs are numeric (e.g. 12345).
Passing them as integers however, causes the Discord bridge's YAML parser
to parse them as integers and its config schema validation will fail.

Fixes #240 (Github Issue)