Use Linux Australia for release and not pycon

Update the recording and materials release to say Linux Australia, not
Pycon Australia. This generates a migration because the model text has
change - but also because in the 0001_initial migration it appears that
this help text is stored as a byte string.

This is a bit weird, but realistically running this migration is not
going to cause us any problems so just add it so we don't end up
fighting django along the way.