Add UI for managing a collection's edX endpoints

[gsidebo]

PR #861 allows admins to set up multiple edX endpoints to which all of a collection's HLS-encoded videos will be posted. For a few reasons outlined on that PR, there was no UI added to manage these endpoints in the front end. This issue covers the work to do that. There are no designs yet, but this can probably be accomplished with a select box (probably not necessary to make it a multi-select) in the collection dialog form (which is used for both adding and editing a collection)