Django's `QueryDict`s are immutable during normal running, and so they
must be copied if any updates are to be made to them. In testing,
though, `request.DATA._mutable` is `True`, so tests can falsely pass.
Introduces a tabbed view component which displays its subviews and
handles Backbone navigation between each tab. This is used to implement
the "My Teams" and "Browse" areas of the Teams tab. Right now the
content of both tabs is stubbed out with a placeholder view which should
be replaced by TNL-1893 and TNL-1892.
