Special notification badge in sidebar for keyword mentions

[zarino]

Building on top of #4 – if a keyword (eg: for now, the user's nick) is mentioned in a background channel, then a second badge should show in the sidebar for that channel, with the total number of missed mentions in that channel.

Focussing the channel should clear the badge, as normal.

The styling for this is already here:

backchat/src/client/sass/_sidebar.scss

Lines 95 to 97 in 4e8a355

	&.important {
	background-color: $color-mac-red;
	color: #fff;