Tags: update mentor enpoint

[yellalena]

• PUT /mentor/:id, POST /mentors receives mentor.tags = [string]

• tags are processed separately and stored into tags_assigned table; if tag didn't exist in tags table yet, create tag in tags table, then assign via assigned_to

• GET mentor returns mentor.tags = [string] (agggregated from tags_assigned table)

• To get tags for mentor (backend): select * from tags_assigned where assigned_id=:id (mentor_id) and assigned_to="mentor" (enum: mentor, study_buddy, project); (+join tags table)

• tests
• swagger