new code is copied stylistically from the email branding patterns.
Instead of `service.dvla_organisation`, there's now
`service.letter_branding` and `service.letter_branding_id`. However,
unlike email branding we're not currently showing a preview of the
logo. That can come later when we work out how we want to do it.
This removes some code which is duplicative and obscure (ie it’s not
very clear why we do `"a" * 73` even though there is a Very Good Reason
for doing so).
We were getting all letter logos from a method in the email branding
client. Since we will be adding more client methods to deal with
letters, it makes things clearer to separate the email and letter
branding clients.
