Calendar with Skype integration

The system was updated to version e5a4d43 today. These are the changes:

New features

Calendar with simple Skype integration that makes it possible for students and supervisors to create appointments between themselves.

MathML embedding was made slightly stricter (TeX support removed) to avoid some potential browser bugs.
Portfolio now uses the more modern HTML5 doctype, which should make browser compatibility slightly better. This also removes all support for XHTML. HTML syntax is now the preferred markup. HTML is no longer cleaned on output, so the "invalid XHTML markup detected" message will no longer be seen.
JSON response mode is now available. All mutable request handlers should now automatically support JSON status feedback.

Deleting groups or institutions associated with PIN code series will no longer give error message.
The previous update made it impossible for institution admins to collect courses. This update restores that functionality.
Collection now uses the same permissions as the other group management features.
Compression of dynamic content on Opera and Safari is disabled because it truncates the end of some JavaScript code.