Extract `mergeable_ci_state` from MergeRequest model into Mergeability::CheckCiStatusService

What does this MR do and why?

This is a follow-up that came out of !165363 (merged). Since mergeable_ci_state is only used in MergeRequests::Mergeability::CheckCiStatusService , we can extract all the related methods into the Mergeability::CheckCiStatusService class so it's cohesive and decoupled from the MergeRequest model.

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#494442 (closed)