Branches page should display open MRs

Description

When displaying the branches on a repository, display the open MRs for each branch.

This makes it possible to determine which branches have already been requested for merge, and which still need to be raised.

Proposal

• Browse to a repo
• Click through to branches
• The open MR, jamietanna/gitlab-test-repo!3 is displayed, instead of 'Merge Request'
• Branches that do not have a MR will have 'Merge Request'

Links / references

N/A

Documentation blurb

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Related to gitlab-ce#33317