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)

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`