Playback formats don't require a playback type, and other small fixes

Allow playback formats to have a nil playback type. It prevents errors if
the data fetched from the web conference server is missing data or has
invalid tags, for example.

Also made BigbluebuttonPlaybackType#name more like the default I18n.t. Now
it returns a "stringified" version of the key searched for if the key does
not exist in the locales.

refs #1142
Add #default to playback types

Allows setting one (and no more) playback type as the default, to be listed
on the top when listing the playback links, to be used as the default for
the BigbluebuttonRecording#play action, and others.

refs #1142
Delegate BigbluebuttonPlaybackFormat#visible? to playback type

refs #1142