Filter notice styling and panel adjustments

* Based on @clarkepaul's commit here:
https://github.com/silverstripe-droptables/sapphire/commit/e5de6c06489f3
449ee8131bf4ee2a4e1b2380bee
* Kept margin adjustments, but avoided implementing tab changes, as
tabs have chnaged a lot since original commit
Merge pull request #2058 from adrexia/filter-panel

Filter notice styling and panel adjustments