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)

It's been pointed out that DEBUG logs could contain sensitive
information (access tokens, etc.), which makes them unsuitable
for sharing with others. INFO should be enough.

Clarify sentence about dedicated user for access token

Also discussed previously in #213 (Github Pull Request).

shared-secret-auth and rest-auth logging is still at `INFO`
intentionally, as user login events seem more important to keep.
Those modules typically don't spam as much.

We recently had someone in the support room who set it to `false`
and the playbook ran without any issues.

This currently seems to yield the same result as 'none', but it's
better to avoid such behavior.

Refer to 524436ebef and #230 (Github Issue).

Fixes #230 (Github Issue).

Related to https://github.com/Half-Shot/matrix-appservice-discord/issues/510

It adds support for a new `DISABLE_SENDER_VERIFICATION` environment
variable that can be used to disable verification of sender addresses.

It doesn't matter for us, but we upgrade to keep up with latest.

Fix apt message: docker doesn't support arch 'i386'

Related to #223 (Github Pull Request)

Update synapse (1.1.0 -> 1.2.0)

Update nginx (1.17.1 -> 1.17.2)