Switch from BSD-3 licence to AGPL-3 licence (#280)

Funkwhale is now licenced under AGPL-3
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Following the recent switch made by PixelFed
(https://github.com/dansup/pixelfed/issues/143), we decided along with
the community to relicence Funkwhale under the AGPL-3 licence. We did this
switch for various reasons:

- This is better aligned with other fediverse software
- It prohibits anyone to distribute closed-source and proprietary forks of Funkwhale

As end users and instance owners, this does not change anything. You can
continue to use Funkwhale exactly as you did before :)