Implements new admin menu structure #304

close #305
Add Users link in drop down menu
Do not require answers if it is a yes or no question.
Fix proposal error message. Username -> name.
add username to deleted_user in seeds
add is_disabled attribute for deleted user
remove leftovers from event attachment removal
Provide a helper to point to sign up/in for different configs
Implements piwik tracking
Merge pull request #504 from ChrisBr/menu

Implements new admin menu structure #304
Moves sponsor email attribute from conference to contact
Merge pull request #503 from ChrisBr/move_sponsor_email

Moves sponsor email attribute from conference to contact
Refactoring rooms
Test for ichain authentication
Refactoring lodgings
Merge pull request #515 from ChrisBr/rooms

Refactoring admin room menu
Merge pull request #530 from ChrisBr/devise

Implements test for ichain authentication
Merge pull request #538 from hennevogel/master

Implements piwik tracking
Refactoring venue
Add clarification when updating user's email