Currently images in our email template are hardcoded to be served from
the live domain[1].
In order for the admin app, running locally or in preview/staging, to be
able to load these images when previewing an email template, the CSP
headers need to allow this domain.
Also splits the header string up using string literal concatenation[2]
so that it’s easier to read.
1. https://notifications.service.gov.uk
2. https://docs.python.org/3/reference/lexical_analysis.html#string-literal-concatenation
Because this commit’s parent added a few new images, we are now serving at
least a handful of images, therefore a few additional HTTP requests. It’s better
to combine multiple HTTP requests into one for performance reasons (up to a
point).
This commit adds an extra step to the preprocessing of SASS files which takes
any images it finds, base64 encodes them and inlines them into the distributed
CSS files.
It also modifies the content security policy to allow inline images.
