Eager load head pipeline projects for MRs index

What does this MR do?

This eager loads the "project" association of each MR's "head_pipeline" association when displaying the MRs index page. This should solve an N+1 query problem that arises when displaying the CI status of MRs.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • 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