Big user database cleanup, performance improvements and various bugfixes

Today's update (b2bba79) includes the following changes:

Performance improvements:

• The My supervisors report should now be much faster.
• The Students and supervisors report (accessible from institution menu) should be a great deal faster.
• The User registration timestamps report (accessible via group view) should be more efficient.

Enhancements:

• Wave files are now played back with on-the-fly conversion to MP3. This should make audio playback of recorded speech more compatible across devices.
• Special user folders are now renamed using the user's framework language during login. The home folder will use the nickname of the logged-in user.
• Error messages are now rendered more consistently, and technical details should be less prominent, increasing usability.
• The CSV parsing of users in the Manage institution handler is more robust.
• The login form and logic has been cleaned up and should be more user-friendly. It is now possible to login using non-HTML workflows (e.g. JSON or XML).
• The link() TT function now allows specifying the size of the popup window. Fullscreen mode is also possible.
• Users registered more than a year ago which have never logged-in have been removed.
• Better explanation of interactivity number on user activity report.

Bugfixes:

• Users with duplicate user names (because of case difference) have now been eliminated and it should no longer be possible to create them.
• When word playback in TTS (used in Mot målet) crosses a line boundary (LF) the sound was previously not played back. This should now be fixed.
• White-space before and after quiz answers (text type) are now trimmed.
• Ensure an invalid avatar user config variable doesn't break user profile rendering.
• Creation of large comments and message responses should now work as expected.
• Config variables are only shown when editing a user's profile if it is your own profile.