Fix border around letter images
Fix border around letter images

Things in CSS stack top to bottom like this:
- image
- `box-shadow`
- `background`

We are drawing the border around the letter using `box-shadow`. This was
working fine because the images of our letters had a transparent
background, so you could see the border through them.

At some point our letters have changed to have a white background. So
you can no longer see the border.

This commit:
- adds a new pseudo element which stacks above the image of the letter
- moves the border from the image itself to said pseudo element

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin