WIP: Show commits by author

What does this MR do?

Allows the projects commit history to be filtered by commit author when clicking on the authors name

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

Currently this feature has not been implemented in Gitaly, as a result this MR only has code that would work for the old way of searching/filtering commits.

Why was this MR needed?

To resolve https://gitlab.com/gitlab-org/gitlab-ce/issues/12760

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #12760 (moved)