Add description text field to data-type model

A human readable description in the meta data for each data-type should help users know what they're for.
Improve schema migration documentation

Our previous documentation was a bit lacking regarding South's schemamigration/migrate commands, and the fact that they should often be used instead of syncdb.
Merge pull request #162 from alphagov/datatype-description-field

Add description text field to data-type model
Merge pull request #163 from alphagov/schema-migration-docs

Improve schema migration documentation
Test that saving an existing data-set doesn't change its name
Saving an existing data-set no longer changes its name

We have older data-sets with names that don't follow the datagroup_datatype naming convention, and their names must not change when they're saved. Only new data-sets should have their names set on save.
Merge pull request #164 from alphagov/modify-doesnt-change-name

Saving an existing data-set no longer changes its name
Migration: create bearer tokens / remove users

A migration that
 - creates new bearer tokens for datasets that do not have
   them so the upload app is able to use the write api
 - removes users from having access to upload to datasets that should be
   collected only
Merge pull request #165 from alphagov/add-bearer-tokens-to-datasets-and-remove-users

Migration: create bearer tokens / remove users
Tests for transactions-by-channel data type
Passing transactions-by-channel tests - add json schema
@WIP stub using pp client to get existing data-sets