Tutor-contrib-branding

- Fix error downloading images

- Add customizable static pages (about, contact us, 404, etc.)

- Add BRANDING_CERTIFICATE_HTML setting to customize certificates
- Add BRANDING_INDEX_ADDITIONAL_HTML setting to add a custom HTML code to the home page

- Allow non-authenticated header to hide links by setting '' to the urls.

- Download fonts and images from URL

- First release