Rename abbrev to short_name

Closes AlekSIS-App-Chronos#70

See merge request !17

Close AlekSIS-App-Chronos#70

Add permissions (via rules)

See merge request !16

Add views for assigning subjects to groups

See merge request !15

Necessary for matching method by subjects and parent groups (MySQL importer)

Resolve "Support import from MySQL"

Closes #4

See merge request !13

Also, throws exceptions on unrecoverable errors to terminate and rollback the transaction cleanly

- Only import substitutions for the current term
- Only import supervision substitutions related to a supervision
- Set subject to None if lesson is cancelled

- Use it for lessons and supervisions