feat: add CookieBanner, CamMuiTheme, + example

For #1 (closed)

What does this MR do?

1. Adds remaining code for CookieBanner in this repo -- it's a component you can use to add an approved cookie banner.
2. It also adds a theme (CamMuiTheme) borrowed from DA.
3. Adds instructions for usage in README.md.

Checks

1. Ran through steps in README.md. All works as expected

2. Compared the cookie banner to what we see in DA (AP frontend) by looking at the prod version:

2. Checked that the code for CookieBanner and CamMuiTheme matches the code in AP frontend.