Style devise views
Insert "user" button in main menu
Wrap nav and menu in header tag
Remove unnecessary rules from global.css.scss
Style and fade out flash messages
Move form stylesheets to global.css.scss
Style and add link to user administration views
Fix 500 error due to unsuccessful login

This fixes some strange devise errors, which seem to occur due to a bug
in / strange behavior of ActionController::Live.
See: https://github.com/plataformatec/devise/issues/2332
Check user permissions for MPD control actions
Show MPD control buttons only to priviledged users and admins
Add example mail configuration