Update Synapse (0.99.2 -> 0.99.3)

Specify that cron is likely required on the server (fixes #125)

When using Let's Encrypt SSL certificates, a cronjob is set up to
automatically renew them. Though it does require a `cron`-compatible
program on the server.

This fixes the error that is caused by the `/etc/cron.d` directory
not existing and the `ansible-cron` module trying to write out a
file there -- without checking if the directory exists first.

By default, `--tags=self-check` no longer validates certificates
when `matrix_ssl_retrieval_method` is set to `self-signed`.

Besides this default, people can also enable/disable validation using the
individual role variables manually.

Fixes #124 (Github Issue)

Discussed in #121 (Github Issue).

Related to #121 (Github Issue)

Most (all?) of our Matrix services are running in the `matrix` network,
so they were safe -- not accessible from Coturn to begin with.

Isolating Coturn into its own network is a security improvement
for people who were starting other services in the default
Docker network. Those services were potentially reachable over the
private Docker network from Coturn.

Discussed in #120 (Github Pull Request)

This is more explicit than hiding it in the role defaults.

People who reuse the roles in their own playbook (and not only) may
incorrectly define `ansible_host` to be a hostname or some local address.

Making it more explicit is more likely to prevent such mistakes.

Add config options to turnserver.conf

Use common path for systemctl in lets encrypt cron

Update homeserver.yaml with some new options we could enable

Doing this for consistency.

Related to #117 (Github Pull Request).

Allow configuring push content for matrix-synapse

Currently the nginx reload cron fails on Debian 9 because the path to
systemctl is /bin/systemctl rather than /usr/bin/systemctl.

CentOS 7 places systemctl in both /bin and /usr/bin, so we can just use
/bin/systemctl as the full path.

This allows overriding the default value for `include_content`. Setting
this to false allows homeserver admins to ensure that message content
isn't sent in the clear through third party servers.

Federation self-check respects matrix_synapse_federation_enabled

`matrix_nginx_proxy_data_path` has always served as a base path,
so we're renaming it to reflect that.

Along with this, we're also introducing a new "data path" variable
(`matrix_nginx_proxy_data_path`), which is really a data path this time.
It's used for storing additional, non-configuration, files related to
matrix-nginx-proxy.

Start appservice-irc as non-root

Add option to enable Dimension widgets serve sites with self signed certs

Fix Matrix homeserver name in Dimension configuration

It's been reported that YAML parsing errors
would occur on certain Ansible/Python combinations for some reason.

It appears that a bare `{{ matrix_dimension_admins }}` would sometimes
yield things like `[u'@user:domain.com', ..]` (note the `u` string prefix).

To prevent such problems, we now explicitly serialize with `|to_json`.

Fix Dimension documentation