Rearrange branding tests to group email / letters
Rearrange branding tests to group email / letters

Previously I found it hard to audit these tests when they flipped
between emails and letters when scrolling vertically.

While these have much in common, they are still separate features
and will evolve separately.

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin