Update related issues/merge requests widgets
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### Issue
- The `Create merge request/branch` button is super useful, but its current placement seems odd and it's overly prominent on the page
- It's currently sort of floating to the right
- On edit, it's [easily confused](https://gitlab.com/gitlab-org/gitlab/-/issues/378018#note_1158588720) with the `Save changes` button
- The other widgets (related issues, tasks, etc) are collapsible and have a action buttons; the MR and branch ones do not
- We have two separate widgets for related MRs and branches; could we combine them?
### Proposal
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