Add 'pyflakes' to PIP_DEVEL requirements.

Required by 'staticcheck' management command.
Handle non existing sessions or users.
Improve management commands help text.
Remove unused imports: staticcheck warnings (pyflakes)
Document restarting services after changing tinc (#388)
fix typo
Implement disk available based on resources member (#559)
Cosmetics: make StateAdmin changelist more user friendly.

- Include content_type info and changelist link.
- Include object change_view link.
- Implement filtering and sorting.
Refactor disk_available.

Conflicts:
	controller/__init__.py
Cosmetics: fix padding of disk avail column.
Configure CELERYD_GROUP via command option (#306)
Add support for proxy when calling pip (#302)
Clean up old commands.
Fix resources permissions (#558)
Implement REST API pagination (#409)

See specifications at:
http://wiki.confine-project.eu/arch:rest-api#pagination
Validate user input: page and per_page.
Add Pagination unit tests.
Extend REST API pagination & filtering tests.
Log sliver history on the node (#523).
Handle ping NoReverseMatch at /admin/pings/