Improve preferences selection

Reworked logic for the preferences selection to make it more robust. Leaving the page when preferences are modified but not yet saved now shows an "are you sure" message. The save buttons also turn orange when modifications were made but not saved yet.

Student preferences are now directly modified and show better error messages.

Furthermore:

• Reuse logic between experiment_preferences and project_preferences
• Generalize logic for student_preferences
• Add new banner for experiments students did not see yet
• Fix permission checks in preferences controllers (prevent server-side edits when they should not be possible any more)
• Fix student can delete preferences of another student

Closes #33 (closed) Closes #100 (closed)