Currently, anyone with write permissions to a repo are able to rename
default or protected branches.
This change follows
[GitHub's](https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-branches-in-your-repository/renaming-a-branch)
design by only allowing repo/site admins to change these branches.
However, it also follows are current design for protected branches and
only allows admins to modify branch names == branch protection rule
names. Glob-based rules cannot be renamed by anyone (as was already the
case, but we now catch `ErrBranchIsProtected` which we previously did
not catch, throwing a 500).
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