Initial implementation of collection sharing with Contributors
-Added Chosen for autocomplete multi-select of users for Contributors (skins a normal multiple select):
https://github.com/harvesthq/chosen
This should scale easily up to 1000s of users.
-Implemented form access/edit and delete permissions for Owners/Contributors
-Owners only have control over private/public of Collection also.

TODO:
-testing
-email notifications?
-listing of who is in group to other collaborators
- fixed ownership of contributors in add form
- moved util functions to util
- fixed folder name bug in upload_folder (also fixed in enh/afni4d)
fixed my collection for contributor
added templated email notifications
added the actual things...
serialized contributor names for api
fixed collection sharing bug for new collections
Filtered owner from collection owners.  Migrated related name for Contributors.
added unit test for collection sharing
-simulates account creation, and browser sessions
-asserts collection sharing permissions work as intended
-the test triggers some warning with crispy forms, since the objects are not fully populated, but it doesn't affect the test case
form changes to support Contributors as comma-seperated input
Fix crispy form bugs with new collection sharing widget
-when a modelform is explicitly extended with a static form field, the field ignores the Meta propertys of the parent Form
-this breaks the exclusion of the owner-only fields, so the inheritance of the Collection form had to be reimplemented. :/
private flag can't be blank
manually merged upstream master (tests pass.  yay CI ;)
Merge pull request #42 from infocortex/enh/collection_sharing

Enh/collection sharing