Add Support for matrix-appservice-discord

Generates the link required to add the Bridge to a Discord server.

Bridge is setup to work on the matrix side with this, but the discord invite link is not automatically generated.

Fixes #69 (Github Issue)

fixed a small path typo in playbook telemetry docs

The Server spec says that redirects should be followed for
`/.well-known/matrix/server`. So we follow them.

The Client-Server specs doesn't mention redirects, so we don't
follow redirects there.

Using `docker_container` with a `cap_drop` argument requires
Ansible >=2.7.

We want to support older versions too (2.4), so we either need to
stop invoking it with `cap_drop` (insecure), or just stop using
the module altogether.

Since it was suffering from other bugs too (not deleting containers
on failure), we've decided to remove `docker_container` usage completely.

Some resources shouldn't be cached right now,
as per https://github.com/vector-im/riot-web/pull/8702

(note all of the suggestions from that pull request were applied,
because some of them do not seem relevant - no such files)

Fixes #98 (Github Issue)

It's not important for us, as it only contains
some ACME-related fix.

`matrix_synapse_no_tls` is now implicit, so we've gotten rid of it.

The `homeserver.yaml.j2` template has been synchronized with the
configuration generated by Synapse v0.99.1 (some new options
are present, etc.)

This reverts commit 536c85619f.

Looks like binding metrics on IPv6 (`::`) fails with an error:

socket.gaierror: [Errno -2] Name does not resolve

We had something like that on the Server Delegation how-to page,
but it's better if we have it on the SSL certificates page.

Relocated there and improved linking.

Fixes #94 (Github Issue)

We don't provide certificates, so it fails anyway,
but we'd rather suppress the warnings about it too.

For consistency with all our other listeners,
we make this one bind on the `::` address too
(both IPv4 and IPv6).

Additional details are in #91 (Github Pull Request).

Add Synapse v0.99 support