Compress the contents of backup files

When discussing !756 (closed) @ayufan suggested to avoid all the issues of having a changing file format and filename of the backup file by compressing the contents instead of the final result.

We should start by only compressing the SQL dump, preferably by piping through gzip. So in the final tar file you find db/database.sql.gz.

When we change this we also need to update the mysql conversion documentation.

@ayufan would you be able to pick up this issue? cc @sytses

mentioned in merge request !756 (closed)

@jacobvosmaer Sure, I will be able to do it at the beginning of next week.

Reassigned to @ayufan

Milestone changed to 7.13

@jacobvosmaer https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/873

Status changed to closed

mentioned in issue #33838 (moved)

mentioned in merge request !14785