create_link testing and cleanup.

- Test various link creation scenarios.
- Show better error messages during link creation.
- Don’t needlessly prefetch entire contents of link.
- Refuse to archive the standard reserved IP ranges.
Fix requirement.
Exclude some non-production files from test coverage.
Include mirroring tasks.
Fix tests.
Test file upload.
Test account creation.
Test link browsing.
Merge pull request #597 from jcushman/dev-create_link_testing

create_link testing and cleanup.