Aa-forum

Added

- Child boards (1 Level). Child boards will inherit their access restrictions from
their parents.

Fixed

- \[Bootstrap\] CSS class moved to the right element

Changed

- CKEditor config changed to prevent possible collisions in static files (see
[README.md] for details)

Added

- Ability to change the subject of the topic (User needs to be the original poster or
forum admin to change a topic's subject)

Changed

- Link to a message now has the full path (category, board and topic) in its URL

Fixed

- When the first message is deleted, the topic will be deleted as well. This is to
prevent having topics with replies to a message that does no longer exist.

Added

- YouTube embed plugin for CKEditor

Changed

- Default config for CKEditor (see [README.md] for details) to implement the
YouTube embed plugin and to remove the obsolete Flash embed

Changed

- Setup classifier and pyupgrade check, since we aim to support AA 2.8.x with its
current minimum Python version of 3.6