Feature/collection index add deck

This PR adds an *ADD A DECK* button to the collection view(s). This makes it more consistent with the interfaces for adding a course and card.

In addition to implementing the above functionality, this PR also merges the `collection_index.html` and `current_collection_view.html`. The templates were nearly identical and there was a lot of repeated markup and code.

@jazahn can you review the code?

@roderick can you review the css/dom changes?
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Jira: [FLASH-55](https://jira.huit.harvard.edu/browse/FLASH-55).