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
