Batch 1 of text improvements

Batch 1 of changes from my effort at !635 to walk through every piece of text in GitLab and see if it can be improved.

This batch includes:

- Improve text on error pages.
- Improve Git access error messages.
- Improve description of branch protection levels.
- Improve OAuth signup error message.
- Improve OAuth application flash messages.

cc @rspeicher

See merge request !642