Keeping consistent between UI and API about combined commit status state and fix some bugs

[lunny]

Extract from #34531

Move Commit status state to a standalone package

Move the state from structs to commitstatus package. It also introduce CommitStatusStates so that the combine function could be used from UI and API logic.

Combined commit status Changed

This PR will follow Github's combined commit status. Before this PR, every commit status could be a combined one.
According to https://docs.github.com/en/rest/commits/statuses?apiVersion=2022-11-28#get-the-combined-status-for-a-specific-reference

Additionally, a combined state is returned. The state is one of:
failure if any of the contexts report as error or failure
pending if there are no statuses or a context is pending
success if the latest status for all contexts is success

This PR will follow that rule and remove the NoBetterThan logic. This also fixes the inconsistent between UI and API. In the API convert package, it has implemented this which is different from the UI. It also fixed the missing URL and CommitURL in the API.

CalcCommitStatus return nil if there is no commit statuses

The behavior of CalcCommitStatus is changed. If the parameter commit statuses is empty, it will return nil. The reference places should check the returned value themselves.