Categories: Change message when no categories

* When there are no source categories, no widgets are displayed
  in either the source or the translation column.
* When there are source categories but no categories can be adapted,
  the message "No categories" is displayed in both the counter and
  listing widgets in the translation column.
* Small css change to make disabled source categories more visible

Bug: 66666
Change-Id: Ie848eb79463841b9f3161bba1ae00b7ab3b1a9ed