Adds basis for Firm matching

* Use factories for `Principal`
* Adds validation summary
* Adds validators for `Lookup::Firm` matching
Shows the type of message as the heading
Completes the FCA number matching scenario

* Rejigs some of the translation entries
Shorthands FactoryGirl `create` calls

* Removes superfluous new line
* Reworks to take advantage of `let` in specs
Merge pull request #35 from moneyadviceservice/4848-non-matched-fca-number

Principal provides a non-matched FCA number
Adds `letter_opener` for previewing emails
Merge pull request #36 from moneyadviceservice/use-letter-opener

Adds `letter_opener` for previewing emails
Replace divs with Dough form helpers
Add form validation summary and error fields