#432 Maintenance Mode: Hide update links to normal users

[Felix]

Currently, when a database update is necessary the links "start update" (or as well "convert testruns") are displayed to everyone who accesses testmaker.
These should only be visible to admin users.

A possible solution would be a dummy page in maintenance-mode (checkbox?) which is displayed on testmaker-access and allows no user interaction at all.
Admins should then have a seperate link, where they can update tM in the background. (something that requires no direct database access, since the database is being updated..)

[Felix]

Implemented a new Extended Maintenance Mode.
This option can be checked underneath the maintenance message and generates a Dummy Page with no user interaction possibilities.
Admins can access testMaker by entering an access-key wich is sent to the enabling admin via email.
This requires cookies to be enabled for admin.