Mentor matching: favor the previous mentor over new mentors

[mpoegel]

Allow users to request a mentor that has previously help them. Due to the time cost and effort of repeatedly explaining the intricate details of a project and problem, this would be very helpful.

Consider:

• adding a checkbox to the mentor request form to request the previous mentor
  • add option to wait if the mentor is currently unavailable
• showing a list of activate mentors so a mentor can be requested by name or company
• update the matching algorithm to favor previous mentors
  • although, we must make sure to avoid a small few mentors doing all the work