Data migration for new moderation datastructure

With #267 (closed) we suggest a new model ModerationSequenceItem. This is how we'd like to migrate existing data:

• RadioReport.{in,ou}tro are removed. If set, a moderation item with moderation_text is created for each of them.
• Audiofile.episode is removed. For audiofiles belonging to an episode we create a RadioReport instance for those files. An existing moderation sequence for the episode will be nested in a single new item containing a reference to the audiofile. (We need to check for the case of more than one audiofile for an episode which is not just a transcoded version of the first.) (See #270 (closed))
• EpisodeItem is removed. (See #269 (closed)) The data is moved into moderation sequence items as follows:
  • internalnote is moved to internal_note
  • moderation is moved to a nested moderation sequence, see below
  • report doesn't need to be migrated, does it?
  • Maybe we want to create a counterpart for the time attribute in ModerationSequenceItem.
• A Moderation will be discarded if it is not used in any RadioReport oder Episode. Referenced moderations will be migrated as follows:
  • name is moved to internal_note
  • note can be migrated to a moderation item with an internal_note (if set)
  • length will be discarded (so far)
  • We have no concept for contributions (RadioReport) with more than one moderation yet. Do we really need this in the beginning?
• A ModerationItem is migrated to a ModerationSequenceItem:
  • content goes into moderation_text
  • folder and name go into internal_note
  • audiofile (see it? ;))
  • length is discarded (Doesn't Audiofile already contain a length attribute?)

We might even want to merge Episode into RadioReport - would be nice, but we leave it for the future.