[#69449568] Add in help text to data-set form

- First stab at explaining all of the fields in the data-set form
- Uses `help_text` on the data_set model, we could do this differently in future.
Improvments to copy and help text

- GOV.UK not gov.uk (be on brand yo)
- visitors not visiors
- journeys not journies
- Be more explicit about leaving the Queryable field alone
- Add data group first warning
Merge pull request #115 from alphagov/add_data_set_help_text

[#69449568] Add in help text to data-set form