[#6829] Make activity timeline look better

Along with style changes, this also includes adding a 'summary' key
to the activity_extras dict for several ActivityObject subclasses,
and adding an 'icon_url' key to the User activity_extras dict.

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>
[#6829] Minor fixes, added tests

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>