Fix #781: Added a `check_inplace_files` management command to remove purge the...

Closes #781

See merge request funkwhale/funkwhale!707

Fix #781: Added a `check_inplace_files` management command to remove purge the database from references to in-place imported files that don't exist on disk anymore

Fix #787: federation issue due to missing error handling

Closes #787

See merge request funkwhale/funkwhale!706

Fix #788: fixed crashing import with empty album artist

Closes #788

See merge request funkwhale/funkwhale!705

Fix #783: migration issue on develop

Closes #783

See merge request funkwhale/funkwhale!704

Resolve "Upload problem due to a formatting error in the date tag"

Closes #718, #583, #501, #252, #544 (yes, that's a lot :D)

So, this MR changes the way we parse audio metadata and handle validation for this data:

- There is now a strict and explicit separation between the retrieve step (extracting raw metadata value from files), and the validation step (ensure the raw metadata is actually valid, convert it to our internal types, etc.)
- The date parsing issue from #718 should be solved (and Funkwhale should not crash anymore against invalid dates, skipping those instead)
- We are now ready to deal with multiple artists at the metadata parsing level (cf !515). It should also be easier to exclude featuring artists now (cf #736), but it's not implemented here
- When invalid metadata makes the import fail, we store the errors on the upload so we can display those to the user
- Tables that list uploads in the Web UI leverage this new data to display more detailed errors and advices, as shown on the screenshot below

## Link to the detail modal

![image](/uploads/20eab969c7cb46fe68a402d8a509815d/image.png)

## Import error detail

![image](/uploads/7cebae6dd05f437345fa38340e8675c7/image.png)

![image](/uploads/946c8ae59cd89fe29e701700fbd5063f/image.png)

As you can see, we display the type of errors, the failing tags and the actual error, as well as some link to the documentation, support forum and some debug info you can easily copy-paste to get help.

See merge request funkwhale/funkwhale!703

Fix #777: Added a prune_library management command to remove obsolete metadata

Closes #777

See merge request funkwhale/funkwhale!698

Updated forum urls

See merge request funkwhale/funkwhale!697

Fix #776: Don't store unhandled ActivityPub messages in database

Closes #776

See merge request funkwhale/funkwhale!696

Fix #740: Do not consider tracks as duplicates during import if they have different positions

Closes #740

See merge request funkwhale/funkwhale!695

Fix #772: Prevent skipping on file import if album_mbid is different

Closes #772

See merge request funkwhale/funkwhale!694

Revert "Merge branch '769-ci-changes' into 'develop'"

See merge request funkwhale/funkwhale!691

This reverts merge request !689

Fix #769: run CI jobs only when relevant

Closes #769

See merge request funkwhale/funkwhale!689

Resolve "Implement a Oauth provider in Funkwhale"

Closes #752

See merge request funkwhale/funkwhale!672

Resolve "Show 'description' of libraries in card"

See merge request funkwhale/funkwhale!688

Fix #570: Exclude in-place imported files from quota computation

See merge request funkwhale/funkwhale!687

Updated mono-container documentation with compose file and NESTED_PROXY

See merge request funkwhale/funkwhale!686