Remove Mentoring

[billdybas]

We currently do not use the mentoring features implemented in the API, and it has become impractical to write tests and maintain those features if they will never be used.

Anything to do with mentoring (headcounts, mentors, shifts, specialties) should be removed.

Since this is technically a breaking change, once done, we should update the API version to be v3.

See: #66

[vuzDylan]

Seem good now that I look at the node-api (its a giant monolith) In the future if we make a mentoring app it should have its own backend

[billdybas]

Note, there are shifts and terms tables that don't have an associated model.