Users are able to manage their details

A bit of a meaty PR to allow users to edit their details (name, email, password, post code and newsletter subscription) and have it all synced with the CRM. 

Notes:

- Removed `registration` and `sign_in` feature toggles as these features are already live. (remove these from your `features.yml`. 
- Cleaned up specs relying on the two toggles above
- PR to update Scripts repo to reflect these changes https://github.com/moneyadviceservice/scripts/pull/129

Just waiting on the Welsh translations for some of the copy. @moneyadviceservice/team-doge 

Prefer single-quoted strings when you don't need string interpolation or special symbols.