Make emails not look like utter garbage

[HappyNTH]

Is your feature request related to a problem? Please describe.

Emails we send currently look and feel awful: we should fix that!

Describe the solution you'd like

Emails should look and feel a lot nicer than they currently are. We should remove currently clashing and inaccessible background/text colours, better add our branding, and ensure sign-offs etc. are not duplicated.

Additional Context

A recently sent email:

Implementation Steps

• Update basic styling of emails, such as setting background & text colours, adding logos etc.
• Audit the templates of all emails we currently automatically send
• Make clear the format emails take on Django when it comes to manual email sends, so sections don't get inadvertently duplicated.