Cache user mentions when they are retrieved from the server.
This logic currently isn't being utilized by the UI. Additionally,
the app should use a 'since' value to only get mentions newer
than those stored in the DB, to avoid excessive web requests.
This will be implemented in a future commit.
User colors are now stored in cache. This is consistent
with other similar variables. I think there's a bug
right now where it doesn't properly refresh colors
for the TimeLineItem when the theme is changed.
