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)

Bump postgres versions (11.5, 10.10, 9.6.15)

This only gets triggered if:
- the Synapse role is used standalone and the default values are used
- the whole playbook is used, with `matrix_mxisd_enabled: false`

Added possibility to enable guest access on synapse

Continuation of #234 (Github Pull Request).

I had unintentionally updated the documentation for the feature,
saying the page is available at `https://matrix.DOMAIN/nginx_status`.

Looks like it wasn't the case, going against my expectations.

I'm correcting this with this patch.
The status page is being made available on both HTTP and HTTPS.
Serving over HTTP is likely necessary for services like
Longview
(https://www.linode.com/docs/platform/longview/longview-app-for-nginx/)

Serve nginx_status to matrix.DOMAIN

Update riot-web (1.3.0 -> 1.3.2)