- 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`