Visual tidy-ups for release

Header notification box too high.
QA:
- drag wordpress to the canvas
- click deploy
- drag a new wordpress service to the canvas
- click deploy
- a notification should appear that is slightly overlapping the header

Fixed sticky headers position when on search view.
QA:
- search for a charm in the sidebar
- check that the sticky headers appear at the top of the charm list

Removed border on last category in search dropdown.
QA:
- click inside the searchbox
- check that the last category does not have a bottom border

I also did a bunch of cleanup in the bws-searchbox.less file as it was a complete mess.