Add support for a banner image for the course

[djplaner]

Some folk like having a banner image for a course.

Modify Collections to

• allow the choice to include a banner image above the collections navigation bar
• possibly in different places eventually
• specify some text that is overlaid on the banner image

tasks

• [ ] Plan out functionality and implementation
• [ ] Implement
• [ ] Document

[djplaner]

Functionality

user can specify

• Link for banner image
  • Collections advises if image has problem dimensions
• how to fit the image into the space available (similar to banner image for modules)
• text to overlay onto the image - perhaps two levels
• location on the page for the banner to appear
  • directly over Collections nav bar the obvious spot
  • What about other locations - as the first element within div#content (used by whyframe)
• which pages the banner should be displayed on (by default just modules)