Clean carrierwave tempfiles

What does this MR do?

It adds a cron job for cleaning tempfiles created by carrierwave gem.

Are there points in the code the reviewer needs to double check?

Carrierwave has its own method to clean tempfiles, but it works only if cache_dir is set by default.
In GitLab cache_dir is being redefined per project in file_uploader.rb so CarrierWave.clean_cached_files! doesn't work.

Why was this MR needed?

Tempfiles are created every time when a file is being uploaded.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #28540 (closed)

added 2 commits

• a600169d - Reverted changes in README
• 2b63a3df - Added merge request to changelog entry

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added 176 commits

• 2b63a3df...e7a64ec3 - 175 commits from branch gitlab-org:master
• 1a32fd2d - Add cron job to clean carrierwave's temfiles

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@rspeicher, could you review the code, please?

added 6 commits

• bffb0c87 - Changed README.md
• f115595d - Add cron job to clean carrierwave's temfiles
• a600169d - Reverted changes in README
• 2b63a3df - Added merge request to changelog entry
• 8a5200ce - Added with_deleted scope to Project
• 40807295 - Merge branch 'clean_carrierwave_tempfiles' of gitlab.com:blackst0ne/gitlab-ce in…

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added 1 commit

• a5f98d93 - each changed to find_each

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resolved all discussions

Current implementation takes all existed projects in DB and removes their tempfiles.
What should we do with existed tempfiles for already deleted projects?

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unmarked as a Work In Progress

Can anyone please review this MR?

assigned to @rymai

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• 6ba7077c - Changed scopes

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resolved all discussions

assigned to @blackst0ne

@rymai, if we decide to use carrierwave's built-in cleaning method, the old cache_dirs should be removed.
I see two ways to get this:

1. Add an initializer where those directories will be checked and removed. This initializer should exist some releases because there are users who don't upgrade their gitlab instances every release. We'll remove that initializer later.
2. Let users remove that directories by themselves by pointing that out in upgrade to 9.0 release document as a separate item of upgrading process.

@rymai, I can't got further until you tell me what the right way it is as a GitLab core member. =)