Add filter to show unused media files

[MizukiTemma]

Short description

This PR adds a filter function to show unused media files in the media library and a description in side bar of media files where the file is used.

Proposed changes

• Add a button for the filter in the media library.
• Use link checker as mentioned in the issue description.
• Get all the media files that are used neither as icon nor embedded item in contents per ajax and show them.
• Add a new description in the file side bar to tell in which content the file is used.

Side effects

• None (i hope)

• In region media library currently, only files that belong to the region will be checked and filtered, becuase region users cannot change or delete global files. Should global files too be filtered shown in region media library?

Resolved issues

Fixes: #1005

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Code Climate has analyzed commit d5237e2 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 76.4% (50% is the threshold).

This pull request will bring the total coverage in the repository to 75.6% (0.0% change).

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[JoeyStk]

Thank you a lot!
In general I think this feature works well. When I use media inside a page, the filter acts correctly and doesn't show the media anymore - this goes both for the regional and the global library. But I still made a few observations:

• Maybe I'm doing something wrong, but I wasn't able to uncheck the filter and think it would be good, if it would also be possible to reset the filter after using it once. So far it is only possible when I remove the filter-unused-media-files from the URL
• This is probably unrelated to this PR, but when I try to access the media library from the editor the loading time was very long (three times it took forever until I restarted the server).
• I have one suggestion for a different translation

I think this is it for now :)

[MizukiTemma]

@JoeyStk

• Maybe I'm doing something wrong, but I wasn't able to uncheck the filter and think it would be good, if it would also be possible to reset the filter after using it once. So far it is only possible when I remove the filter-unused-media-files from the URL

I've introduced an navigation titling "Home > Unused media files" like when a search funcion is being used. so users can go back to the home directory where all the files are shown just in the same way with after key word search.

• This is probably unrelated to this PR, but when I try to access the media library from the editor the loading time was very long (three times it took forever until I restarted the server).

I think, as you meant, this is not because of PR, for I've seen this problem before this PR sometimes. If you think this cannot be ignored anymore, we can have a closer look at it 😃

• I have one suggestion for a different translation

Applied!

[timobrembeck]

Thanks a lot! 👍

Apart from my comments below, I'd also suggest to disable the "Upload" and "Create directory" buttons when the filter is applied, since it might not be clear to which directories the new files are uploaded.

[timobrembeck]

Thanks a lot! 👍

I refactored the structure "a bit", mostly to improve performance and to somehow implement my suggestion of showing the usages in the sidebar (even though I did not manage to find a query that is fast enough to include it on every request, so I added an additional ajax step).

So from my side this is good to go, but could you have a detailed look again at my changes and check whether I made some other mistakes somewhere? Thanks!

[JoeyStk]

Thank you very much for all the changes. It looks a lot better now 😄

[MizukiTemma]

@timoludwig

I really appreciate your suggestions and detailed explanation 🥺 🙏 it's full of improvements and I find nothing bad 😃