Add offer data migration to ensure no null descriptions

As reported by @riklaunim, the migration that makes offer description
non-NULLable can cause problems if you already have offers with null
descriptions.

Unfortunately, the only way around this is to insert a new datamigration
before the old one, and to renumber all later migrations.  That's what
is done here.

The side-effects for people running against master might be quite
painful as they will need to tinker with their South database tables to
ensure things are consistent.  Probably best to remove all entries from
0020 onwards then fake everything.  Apologies all round - however, this
is the better long-term fix.

Fixes #977