Avoid same user or multiple requests

[lorepirri]

Problem

Currently we allow a user to perform multiple requests on one other user, and also on the same user (the logged in user).

Feature Description

• [x] Do not list a Mentor if a request is already issued to that user
• [x] Do not list the same user among Mentors
• [x] api endpoint request:
  • [x] do not accept requests from the same user
  • [x] do not accept requests for a Mentor if there are already other active requests from the same user