Display a clear security warning after import
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### Problem Description
This has been [discovered](https://gitlab.com/gitlab-org/manage/import/discussions/-/issues/42#note_1244837538) during the competition gaps review in https://gitlab.com/gitlab-org/manage/import/discussions/-/issues/42
GitHub displays this warning immediately after loading the page for the newly imported repo:

This was interesting, given our discussion about protected branches and the like.
### Proposal
When an import is completed and the project contains unprotected branches, let's display a warning on the project overview and the option to immediately protect the branch.
issue