Mappings CSV export

We don't want to encourage use of spreadsheets, but we do occasionally get
asked to provide them. Let's make life easy on ourselves by giving us a means
to provide the spreadsheet easily. It also enables advanced users to search and
filter in ways we don't intend to support (eg look for mappings matching a
regex).
We accumulating a lot of different `redirect_to` uses and methods to do with redirecting in various ways in this controller - do we really need this variety of behaviours, or could they be simplified to a few options? Eg:

https://github.com/alphagov/transition/blob/mappings_csv_export/app/controllers/mappings_controller.rb#L26
https://github.com/alphagov/transition/blob/mappings_csv_export/app/controllers/mappings_controller.rb#L54
https://github.com/alphagov/transition/blob/mappings_csv_export/app/controllers/mappings_controller.rb#L116
https://github.com/alphagov/transition/blob/mappings_csv_export/app/controllers/mappings_controller.rb#L127
https://github.com/alphagov/transition/blob/mappings_csv_export/app/controllers/mappings_controller.rb#L132
https://github.com/alphagov/transition/blob/mappings_csv_export/app/controllers/mappings_controller.rb#L134
https://github.com/alphagov/transition/blob/mappings_csv_export/app/controllers/mappings_controller.rb#L175

(and a bunch of `render`s as well...)

This doesn't need to be addressed as part of this PR, but we should take a look at whether those are still doing the expected thing and if doing fewer different things would be less confusing for users.