rcfilters: Style fixes and cleanup

rcfilters: Style fixes and cleanup

* Set border-box on FilterGroupWidget and FilterItemWidget to prevent
  horizontal scrolling in the filter popup
* Remove unused styles for -invalid-notice and -invalid-filter
* Move styles for FilterCapsuleMultiselectWidget to the right file
* Suppress both bottom and top margin for FieldLayouts in FilterItemWidget
  (the only reason a top margin wasn't applied was because the FieldLayout
  was a first child, which is somewhat fragile)

Change-Id: I7f08ca6aa13fe6429c7fbe45b3d698c20b8c1275