Improve Docker installation documentation

See merge request funkwhale/funkwhale!810

- Use `useradd` features to simplify user creation:
  - `-U` creates a user group,
  - `-G docker` adds the user to the `docker` group
- Document available versions and their usage
- Use `chmod 0600` before editing the file!
- Use `cat` instead of multiple `echo`es
- Comment all lines in `.env`
- Use `${FUNKWHALE_VERSION}` where applicable instead of `|version|` to
enable copy-pasting more commands

Implement tag models

See merge request funkwhale/funkwhale!814

Fixed typo in source translation

See merge request funkwhale/funkwhale!821

Resolve "Add a "Replace current playlist" option"

See merge request funkwhale/funkwhale!820

Fix #877: Ensure API urls answer with and without a trailing slash

Closes #877

See merge request funkwhale/funkwhale!817

Custom css

See merge request funkwhale/funkwhale!818

Resolve "Provide a default format when transcoding via Subsonic"

See merge request funkwhale/funkwhale!819

Front-end minor fixes / error handling

See merge request funkwhale/funkwhale!816

Fixed flaky tests because of license cache

See merge request funkwhale/funkwhale!815

Pressing return on searchbar to search for artists

See merge request funkwhale/funkwhale!812

Human readable stats

See merge request funkwhale/funkwhale!811

Broken pytest

See merge request funkwhale/funkwhale!813

Resolve "Broken link to artist page in album and track header"

See merge request funkwhale/funkwhale!809

Resolve "Provide a default format when transcoding via Subsonic"

See merge request !807

Fix #867: Added a SUBSONIC_DEFAULT_TRANSCODING_FORMAT env var to support clients that don't provide the format parameter

Resolve "Cannot edit users details"

See merge request !808