Make it possible to erase build content (artifacts, trace)

This feature makes it possible to remove build content - build artifacts and build traces.

- [x] Remove artifacts
- [x] Remove metadata
- [x] Remove build traces
- [x] Wait for https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/1942 this to be merged
- [x] Fix the permissions after the merge

Closes #3421

See merge request !2560

Mention project name in fork notice

_Originally opened at !1186 by @axil._

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I missed this one in !1070

See merge request !2897

Discussion:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2560#note_3742042

Discussion:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2560#note_3659905

[ ci skip]

Also added API specs for that

+ minor improvements