This is a platform admin page which lists all letter brands and their
domain. Once the page to edit a letter brand has been created, each
brand will link to its edit page, but for now this is just a list of
brand names and domains.
Form errors were being shown (such as a domain not being a valid), but
we weren't showing nicely formatted error messages if the database
failed to save a new row.
This has a form with 3 fields - the file upload field, logo name, and an
optional logo domain. Logos need to be uploaded in `.svg` format and we
then convert this to `.png` format and upload both file types to S3 as
well as saving the letter branding details in the database.
