Update bigbluebutton_rails to branch 'branch-v1.4.0'

It now keeps track of the meetings that happened. Uses resque to do so, so
now resque is a dependency of mconf-web as well.
Create recent activities for meetings that happened

Whenever a BigbluebuttonMeeting is created, an observer will create an activity
for it, and it will be shown in the user's recent activity.
Pretty much done, just need to improve it once we have associations between
meetings and recordings in BigbluebuttonRails.

refs #959
Show recordings in meetings shown in recent activity

Updated BigbluebuttonRails to a version that associates meetings with recordings,
so now we can show in the list of recent activity the recording that is
associated with a meeting.

refs #959
Update strings for activity, show proper icon for removed objects

Added an icon that is shown whenever the object of an activity was removed.
The strings were reviewed and updated.