Add metadata fields to Project models to improve discovery & presentation

[alex-ketch]

To allow better discoverability of community projects, and to give ourselves more information to work with when deciding how to order the projects list page (#55), we should allow a mix of derived meta information as well as user-submitted information:

• Project banner (for use in Projects listing view and/or as social open graph image)
• Project subject matter tag/selector, extract from Schema keywords
• Programming language(s) used

Review Creative Work Schema to see what fields can be extracted automatically from the main file.

This data will be utilized:

• in the project gallery view #872
• project details view (the Overview page)
• project search view

[nokome]

Related to banner image: https://github.com/stencila/hub/issues/53