Feature completeness

[Kobzol]

This PR tracks the features of homu implemented in bors.

• Try builds
• Setting PR state
  • PR delegation (feat: add delegation commands #210)
  • PR priority (feat: add priority option #205)
  • Approvals (feat: implement r+, r= and r- #114)
  • Rollup state (Add rollup mode management #221)
  • Tracking base branch (Track base branch #230)
  • Tracking mergeability (Detect unmergeable status of PRs #218)
• Setting repository state
  • Tree closing (Tree Closing and Opening #223)
• Merging pull requests (merge queue)
  • Rollups
  • Retrying merges
• Website
  • Help page
  • Queue page
  • GitHub authentication
  • Rollup creation