branch/MR dependencies tree, lock until merged

I think we could show a branch dependency tree component.

Using this, we could lock MR's that have dependencies that aren't merged. We can also show related MRs that don't lock the current MR but may be relevant.

We can also use this across projects and track EE/CE ports.

Example of

backend (!5) -- MERGED
frontend (!9) -- NOT READY, APPROVED
    └─ backend-api-addition-missed-from-spec (!10) -- NOT READY, CI FAILING
      └─ improved-tests-catching-edge-case (!3) -- READY

We can use this opportunity to improve the existing project-level "Network" (complete commit hierarchy) feature with shared dependency graphing components.