Correct spacing around GOV.UK logo on mailers

This is mainly to fix the [spacing around the GOV.UK logo](https://github.com/ministryofjustice/prison-visits/compare/mailer-spacing?expand=1#diff-10b8abd1ff3bdfaa1fdc1f630a2965a1R6) in the mailer banner. The rest is simply removing some unnecessary spaces/semi-colons and consistent quotes.