backups: Create archive, validate/dump manifests, fixes and tests
- This has changes rebased from the backups-epic branch.
- Leaves backups module disabled for now.
- Select apps to create backup. No longer have to input a path.
- Validate manifests, and include them in the backups.
- Fix several issues in backups.py and add tests.
- This MR doesn't include anything related to restoring backups.