Revert 'presentation' dir to 0.81 code, add 'presentation2' for 0.9

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 0.9 recording scripts to use playback support files
from a different subdirectory, 'presentation2', while existing 0.81
recordings continue to use the 'presentation' directory.

If the raw files from an old recording are reprocessed, it will
switch to using the newer playback support files.
Use the directory presentation/0.9.0 instead of presentation2

This makes it a bit more obvious how the versioning can work going
forwards, and makes it so that we do not need to update the nginx
config each time we have a new version of the playback files.

The 0.81 files remain at the top level of the presentation directory
for compatibility reasons.
Move 0.81 files to a subdir, add nginx redirect for them.

Now the 0.81 presentation playback files are in an appropriate versioned
directory. If desired, the recording metadata can be updated to reference
the versioned files directly, but an nginx redirect has been added so
that unmodified recordings will also work.
Add a note to the nginx config to tell 0.9.0 beta users of a possible upgrade path
Add a script to handle updating links in existing recordings

This adds the version number to the playback links so that
recordings select the correct version-specific playback support
files.

This script may be run automatically during upgrade; in that
case it should be run like

.../bbb-0.9-beta-recording-update --quiet

After copying recordings from an old BigBlueButton server, you
may want to manually re-run the script, and it takes the option
--force to recheck all recordings even if it has previously been
run.
Merge pull request #403 from kepstin/recording-updates-2

Version the playback files for the presentation recording format