Add styling for @mention autocomplete (At.js)

design/custom_amelia.css

design/custom_bootstrap.css

design/custom_cerulean.css

design/custom_cosmo.css

design/custom_cyborg.css

design/custom_flatly.css

design/custom_journal.css

design/custom_lumen.css

design/custom_readable.css

design/custom_simplex.css

design/custom_slate.css

design/custom_spacelab.css

design/custom_superhero.css

design/custom_united.css

design/custom_yeti.css

design/style.css

less/components/dropdowns.less