Version the playback files for the presentation recording format

This fixes an issue where recordings copied from a 0.81 server might not play back correctly on a 0.9 server.

This changes the layout of the 'presentation' directory to have versioned subdirectories for the different BigBlueButton releases, making it easier to preserve backwards compatibility. Newly processed recordings will reference the appropriate versioned directory directly.

An nginx redirect has been added to support placing unmodified 0.81 recordings directly onto a 0.9.0 server; it redirects the playback.html request to the 0.81-specific version of that file.

If the raw files from an old recording are reprocessed, it will switch to using the newer playback support files.

**Note that this change will break all recordings made with previous 0.9 beta versions!**

Recordings made with previous 0.9 beta versions will have to either:
* Reprocess all recordings, _or_
* If the server will never have 0.81 recordings, edit the nginx config to redirect to the 0.9.0 files, _or_
* Edit the metadata.xml in all recordings to change `presentation/playback.html` to `presentation/0.9.0/playback.html`