Only return history information up to the displayed version

History information should not change after a module / collection is
published, so it should not include any information about newer
versions.  (We cache the results when the module / collection is
published)
Move downloads information into extra API

 - Remove exports-allowable-types API
 - extra API should contain information about a collection or module
   that can be changed after it's published
Add is_latest flag to the extra API

is_latest indicates whether the selected module is the latest one
Merge pull request #92 from karenc/history-info

History info and extra metadata API