* [CI] Run stylelint
Perform some basic CSS sanity checking and style enforcement.
I removed vendor prefixed linear-gradient and transform because those are from the IE9 era. With IE11 as a minimum and soon obsolete requirement it doesn't make much sense anymore.
* Remove as-link override
* Don't require newline after comment
* Also apply those newline rules to SCSS
* refine opening/closing braces, allow for single-line
* Change category configuration
Before, we had a drop-down list to interract on categories. It was not
working the same way as feeds.
Now, categories and feeds behave in a similar manner. At the moment,
there is no change in features but it will allow to expand them.
See #2369
* Minor whitespace
