Remove unused strings from locales

refs #1147
Update bigbluebutton_rails and improve names for playback types

Also added code to generate the new structure of playback types on
db:populate.

refs #1147, refs #1142
Update bigbluebutton_rails to improve playback links

Order playback links, showing the default always at the top. Include the
duration only for the first playback link, since they should all have the
same duration. Also improved a bit the dates shown with the recordings.

refs #1142
View and controller to edit playback types

Overriding the controller from BigbluebuttonRails to implement our own
authorization restrictions and a new action #index to show all playback
types and allow admins to edit them.

Includes an update on BigbluebuttonRails

refs #1142