Allow subclasses of RB.CollectionView to specify the item container element.

Review Request #10740 — Created Oct. 2, 2019 and submitted — Latest diff uploaded

Review Board

RB.CollectionView makes automatic rendering of items from a collection
easy, but it has a limitation in that all items render to the
collection's main element. This prevents subclasses from having a more
complex element tree.

This updates RB.CollectionView to introduce a new $container
attribute, which is used for all rendering. It defauls to $el, but
subclasses or consumers can override it to point to another element in
the view.

Unit tests passed.

Successfully used this new attribute in an upcoming change.