feat: track mergeability

[Sakib25800]

Related issue: #218

This PR tracks merge ability. It adds a new column: merge_state.

Homu Implementation

• Mergeable status is tracked via the mergeable column in the pull table
• On PR open / reopen / edit / push: mergeable is set to None
• On PR edit: mergeable is set to None only if base branch has changed
• On branch push event:
  • All PRs targeting the given branch have merge_state set to None
  • If still null, then wait 5s
  • If still null, then add to queue
• Async: all PRs with a mergeable of None are scheduled to refetch in a background task via a queue

Proposed Implementation

• Mergeable status is tracked via the merge_state column in the pull_request table
• On PR open / reopen / push: merge_state is set to unknown
• On PR edit: merge_state is set based on payload value
  • The reason we don't do this only if base branch changes is because the payload value can actually be something else other than unknown, whereas with the other events it is always unknown (due to the nature of the events)
• On branch push event: all PRs targeting the given branch have merge_state set to unknown
• Async: needs further thought -> left for another PR

[Sakib25800]

@Kobzol is the goal now just to track merge ability, more specifically mergeable_state in the database?

[Kobzol]

Yes, I think that is a good start :)

[Sakib25800]

Due to this PR becoming an absolute beast, part of it has been split - see #230.

Once that has been merged, this one will continue on from there.

[Kobzol]

Thank you for the PR! Left a few comments.

[Kobzol]

Thank you! Please squash and we can merge it.

[Sakib25800]

Squashed.

[Kobzol]

Thank you! I modified the PR description, as this doesn't fully fix #218 yet.