CFD-139: Custom browser for group banner image

#208 

This is a follow up for the PR #217 
The banner content type is only used to have an easy/cheap interface (for platform admins) to manage the predefined images. 

The banner image should be global content > not group content.

No need for these fields:
- c4m_body
- c4m_vocab_date
- c4m_vocab_language
- og_group_ref
- og_vocabulary