Swapped ng-keydown to a directive, which does the same thing (check if up/down
arrow then call history.goUp/goDown). This has *dramatically* improved
performance when typing in rooms which have lots (>100) of messages loaded.
The first red blink was caused by an uninitialised search index. There is no
caching of entries, since this then wouldn't update if someone joined/left
during the tab. Instead, set to search index to MAX_VALUE then fix it to a
valid index AFTER the search is complete. Also ditched trailing space on ": ".
