πŸ’„ Cosmetic CSS tweaks to make @mention autocomplete tooltip prettier

Added box shadow, border radius, Droid font, and a slightly different shade of grey for arrow key selection vs mouse selection (nano UI to let users know when mouse cursor overlaps with arrow key selection).

Tried adding an up caret to the tooltip to point the tooltip to where the username will be inserted, but got tired of trying to unwrangle CSS between overflow:auto and unhiding the up caret. Just want to launch this freaking thing!