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.
Some of the tests were calling methods that didn't exist on MagicMock
objects, which meant that they were always passing. For example, tests
using the format of MagicMock.called_once_with(...) were always passing
since a MagicMock does not have the method `called_once_with`. Changing
this to `assert_called_once_with` ensures that the tests fail if they're
called with the wrong arguments.
Separated s3_client.py into 3 files - for logos, CSV files and the MOU.
This helps to keep things clearer now that we need to add lots more logo
functions for letters.
