This works in a similar way to the create_letter_branding view function,
but with a few minor differences:
* Since we already have a file, uploading a file is no longer required
(since we can just use the current file if a new one is not uploaded)
* We save the changes in the database, then upload the new files to S3.
If saving to S3 raises an error, we now rollback the database changes to
prevent any errors when trying to view letters with the original logo.
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.
