Add a migration to create recent activity entries

The migration creates activities for the 'create' action of some models.
This way the list of recent activity won't look so empty after the migration
to PublicActivity.

refs #906
Fix undefined method 'default' for BigbluebuttonServer

Was hapenning in development only, because rails unloads initializers and
doesn't reaload them again. Fixed adding the initializer inside a 'to_prepare'
block.
Note that it won't work with 'ActiveSupport.on_load(:after_initialize)' as
done in some other initializers.

More at: http://stackoverflow.com/questions/5852580/why-do-includes-in-rails-engine-initializers-malfunction-when-cache-classes-fahttp://stackoverflow.com/questions/5852580/why-do-includes-in-rails-engine-initializers-malfunction-when-cache-classes-fahttp://stackoverflow.com/questions/5852580/why-do-includes-in-rails-engine-initializers-malfunction-when-cache-classes-fa