Show text saying `Can't approve` if a user can't approve an MR
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### Proposal
Following on from this internal RFH: https://gitlab.com/gitlab-com/request-for-help/-/issues/3198+
When a user is not allowed to approve an MR, they do not see an `Approve` button in the MR widget.
Instead of not showing a button, we should display some text that says something like `Can't approve`, so it doesn't look like a bug with the missing button.
A future iteration could also display the reason _why the user can't approve_.
issue