Upgrade Flot to 0.8.1.

We haven't updated Flot in a while. The newer versions support Retina
displays, which really improves the look of graphs in the admin UI.

None of our code has to change, though we do need to add the flot time
plugin for the Review Board Activity widget, as this is no longer part
of Flot.
Simplify and generalize review UI instantiation.

Review UIs no longer need their own custom templates to set up the
models and views for UIs, or to allocate a location to pack it into.
This is all standardized now in a default review UI template, and all
the customization goes into the ReviewUI subclass.

ReviewUI now has some new fields to set: name, js_view_class,
js_model_class, and js_files. It also introduces get_js_model_data and
get_js_view_data functions.

'name' indicates the displayed name of the review UI, or rather the type
of thing being reviewed. This should generally be something like
'Image', 'PDF', etc.

'js_model_class' and 'js_view_class' are the names of the classes in
JavaScript that are to be instantiated. These are assumed to be
subclasses of AbstractReviewable and AbstractReviewableView.

'js_files' is a list of file paths. This is not fully fleshed out, and
will more than likely be replaced after moving to require.js.

'get_js_model_data' and 'get_js_view_data' return data that will be
serialized and passed to the model or view during instantiation.

Review UIs should be updated to provide these and to remove the
template_name and object_key fields. If they do need something special,
they can override the default template. There are some blocks that can
be used to add fine-grained changes.