Add Jitsi support

We do this for 2 reasons:

- so we can control things which are not controllable using environment
variables (for example `stunServers` in jitsi/web, since we don't wish
to use the hardcoded Google STUN servers if our own Coturn is enabled)

- so playbook variable changes will properly rebuild the configuration.
When using Jitsi environment variables, the configuration is only built
once (the first time) and never rebuilt again. This is not the
consistent with the rest of the playbook and with how Ansible operates.
We're not perfect at it (yet), because we still let the Jitsi containers
generate some files on their own, but we are closer and it should be
good enough for most things.

Related to #415 (Github Pull Request).

Discussed in #412 (Github Issue).

Some options are no longer required and have sensible default values.

Release synapse v.1.12.0

how to activate dimensions

I am new to synapse. Thanks so much for these playbooks!

I wasn't sure how you actual activate dimensions after setting these variables. Should you re run 

ansible-playbook -i inventory/hosts setup.yml --tags=setup-all
ansible-playbook -i inventory/hosts setup.yml --tags=start

? If so perhaps you could tell the readers to do this somewhere in the help file

Thanks =]

Fixes #403 (Github Issue).

Update nginx (1.17.8 -> 1.17.9) and certbot (1.2.0 -> 1.3.0)

Update postgres (12.1 -> 12.2, etc)

Update riot-web (1.5.12 -> 1.5.13)

Fixes #396 (Github Issue).

Related to #386 (Github Pull Request)

Raspberry pi

This keeps the roles cleaner and more independent of matrix-base,
which may be important for people building their own playbook
out of the individual roles and not using the matrix-base role.

Update synapse-janitor to support current synapse database schema

Customization welcome

In relation to #392 (Github Issue)