Store more metadata within the unit

This will allow us faster browsing of units and faster gathering of
stats. Unit data now include status of comments/suggestions/checks
and number of words.
Missing comment
Update words count on save
Update checks flag during runtime
Create mixin class for access to related units for checksum referenced data
Update suggestion stats on runtime
Fix attribute name
Update comment stats on runtime
Fix syntax error
Correctly define type
Data migration for new unit fields

This one is quite lengthy as it needs to go through all units and
process their data.
Use new unit flags while browsing and counting units
Include fields for word counts in translation
Count words when working with translation
Add migration to calculate number of words