Alterations to projects offerings

Closes #764

Adressed points:

• Added sum for indicated supervision capacity (both assigned and indicated).

• When exporting coaches with indicated capacity, add only coaches who indicated capacity (capacity > 0) (this was discussed with Taico)

• Made queries in export coaches and showing groups consistent: groups are called through @course_edition.groups now. Added rejected to statistics (discussed with Taico)

• Fixed notifications in situations when project is deleted.

• Migrated requires_nda to Project, so that it can show diff.

I decided to implemented course roles and client in new PR, as it will require a lot of alterations.