[C3] Database clean-up

[ywarnier]

These are changes that should be applied to the 2.0 database structure to improve it.

• in general, all title fields should be longtext. There's a valid argument that we can have some efficiency gain of assigning smaller variable sizes, but it's probably insignificant in comparison with varchar(255)
• all "url" fields should be longtext. There will always be a case where a URL field of 255 chars is too short
• all varchar fields of a length above 255 should be longtext
• all varchar fields of a length above above 50 should be 255
• course_rel_class.course_code should be course_id int
• the course table has to be cleaned up of all useless fields. Stuff like department_name, department_url, visual_code and tutor_name are deprecated, maybe more (identify the difference between description and introduction and if introduction is not replaced by something in another table, like c_tool_intro or c_course_description)
• check if course_rel_user.sort and user_course_cat can be removed (seem deprecated, this was for personal sorting of courses)
• course_request.category_code should be a category_id int
• remove fos_group and fos_user_user_group (remainders of the implementation of users from a FoS bundle)