Rework category/tags css

[raimund-schluessler]

The css of the categories / tags selection in the detail view is partially broken and needs to be fixed.

[raimund-schluessler]

Also the separate css file in css/vendor/angularui/ui-select/select2.css should be removed and the respective rules should be incorporated in css/style.scss.

[raimund-schluessler]

Fixed with #194.