Determine if person is a mentor/mentee based on whether they provide information on these topics

[igal]

I'm not keen on the person form asking me "Would you like to be a mentor?" and "mentee?" when it could just check to see if I've put anything into the associated textareas to determine this on its own.

This should be a fairly simple change:

• Remove the checkboxes from the person#_form.
• Add Person#mentor? and Person#mentee? and use those instead of the current checks in the person#show form.
• Rework the Person::mentors and ::mentees scopes to return records whose "mentor_topics" and "mentee_topics" are present.

Thoughts?

[reidab]

I'd prefer to see the mentorship checkboxes show/hide the text fields, in order to save form space for people who aren't interested in mentoring