Build namespaces / automatic rebuilds

This issue is a big dump of features which will need to be split up into multiple issues.

• TUI & web UI for showing aggregated build status of multiple packages that depend on one another (#45, #28, #40)
• Every build job is attached to a build namespace, a git commit, and possibly a Gitlab MR
• Automatically include dependent packages in buildsets (#51, #50)
• Allow manually grouping specific builds in buildsets (#41, #62)

The following user stories are superseded by the mockup in #2:

• User story: I want to update opencolorio
  1. pkgctl update --pkgver=1.2.3
  2. This opens MR in the opencolorio repo
  3. MR gets an automatic comment from the system linking to a rebuild that was detected & initiated automatically
  4. Rebuild status can be viewed in a custom web UI or pkgctl TUI or in the automatically updating GitLab comment
  5. The system automatically opens new MRs for all depedents that need to be rebuilt, bumping pkgrel
  6. All builds of all packages succeed, buildset gets marked as overall success
• User story: I want to move a buildset opencolorio-1.2.3 from [extra-staging] to [extra-testing]
  1. pkgctl buildset list then pkgctl buildset move opencolorio-1.2.3 extra-staging extra-testing merges MRs, moves buildsets from ad-hoc repos to testing or staging, and updates pacman databases