Last attempt (08f31a3f) fixed
compilation bug generated a bug because an empty string was generating
an extra --volume parameter on container creation.

Now we move back to using an Array and fix our deploy script
accordingly.

We want to avoid possible problems in a future when more variables are
added to the YAML file and the regular expression becomes too broad and
starts manifesting a bug.

- Move all non-secret variables that are common to different
  `tails::weblate::*` classes to `tails::weblate::params`.

- Make tails::weblate::params inherit from `tails::website::params`.

- Make many classes inherit from the new `tails::weblate::params`.

- Standardize some parameter names accross classes (eg.  `$postgres_*`
  and `$redis_password`).

- Move `$user`/`$group`/`$uid`/`$gid` to
  `$system_user`/`$system_group`/`$system_uid`/`$system_gid`

By having app deployment separate from test suite running we have more
flexbility during development.

This commit also:

  - Moves CI scripts to the Weblate scripts dir, so they now live
    together with the scripts they are supposed to test.

  - Updates the Vagrantfile to use the `tails::weblate` class to deploy
    the application.

  - Updates Weblate hacking instructions.