feat: sanitize media filenames according to global slug setting

[stefk]

Currently, users are allowed to upload images whose filename contains accents, spaces or special characters. This may cause several problems downstream (#857).

It seems that at some point, the work done in #1135 should have fixed that issue (see #857 (comment)) but I can't find any trace of that behaviour in the current codebase.

This PR adds/restores that feature by applying the policy defined in the global slug setting to media inserted via the default media library.

[erezrokah]

Thanks @stefk, this does look like a regression.

[erezrokah]

Not sure why tests are failing on your branch, I will look into it soon

[stefk]

Thanks @erezrokah. I was also puzzled by the test failure, considering everything was green yesterday and that I had just amended a commit this morning with unrelated changes. Then I noticed that the failure was on editorial_workflow_migration_spec_github_backend.js, which wasn't present in my branch (it comes from the recent #3316). Rebasing solved the build issue, but do you have any idea how was this situation possible in the first place? Why was a spec from the master branch executed on a distinct branch?

[erezrokah]

Thanks @erezrokah. I was also puzzled by the test failure, considering everything was green yesterday and that I had just amended a commit this morning with unrelated changes. Then I noticed that the failure was on editorial_workflow_migration_spec_github_backend.js, which wasn't present in my branch (it comes from the recent #3316). Rebasing solved the build issue, but do you have any idea how was this situation possible in first place? Why was a spec from the master branch executed on a distinct branch?

That is a new test and I think it was a bit flaky (hopefully I fixed it).
We actually reverted that PR and re-introduced it so I think you got caught in the middle somewhere.