Hotfix that de-normalises some group data into the tag

Prevents thousands of database queries when rendering tag strings.
Data migration to populate the new fields.
version bump and implemented reverse update from TagGroup to Tag
data migration actually a DataMigration now!
Merge pull request #11 from UKTV/denormalise_group_name

Hotfix that de-normalises some group data into the tag
Merge pull request #12 from UKTV/develop

Develop