Draft: Restructure themes and thesis projects

DB:

• Make an n x n programme - theme relation based on the existing programme - research group - theme relation (the diagram says n x 1 but I'm pretty sure that's wrong)
• Remove the programme - research group relation
• Change the research group - theme relation from 1 x n to n x n
• Change the thesis project - theme relation from 1 x n to n x n

Main UI changes:

• Admin search view (Ransack associations)
• Update admin theme pages
• Update admin thesis project pages
• Update admin research group pages
• Add a way to select research groups on admin theme page
• Add a way to select programmes on admin theme pagel
• Add a way to select themes on admin thesis project page
• Make navigation links on admin panel show pages clickable (e.g. themes of a thesis project)
• Fix dashboard (normal and admin)
• Update student view

Models:

• There must be at least one research group associated with every theme.
• Rename Theme to Topic (only in language files)
  • Still needs to be done in some non-translated strings

Permissions:

• Research group members should be able to make new themes.
• Everyone with access to the theme should be able to link more associated users and research groups.
• Associated users should have access to a theme even if they're not part of a research group. Make sure they have access to the management panel to see the theme.
• Programme coordinators should be able to create themes and thesis projects
• Programme coordinators should be able to access all themes and thesis projects and edit them.
• Only programme coordinators should be able to edit the programme association, and only the association with their own programme.
  • Add message to contact coordinator.

Tests:

• Fix failing tests
• Theme - research group
  • Theme validation (at least one research group)
• Thesis project - theme
• Theme - programme (programme permission checks)
• Permissions

Closes #679