The Unified Review Banner Base Shell sets the foundation for the Unified

Review Banner, which will eventually take over for the three banners 

currently used for draft review requests, reviews, and review replies. 
The banner tracks the draft state of review requests, reviews, and

review replies so it can be put into a draft mode where the banner

appears green when at least one draft exists. The banner makes use of 

RB.FloatingBannerView to allow the banner to float to the top of the

screen when the user scrolls down. The new RB.Registry model allows

views like dropdowns, buttons, and Backbone views to be registered in

different locations on the banner. There are placeholders registered to

the different banner sections right now which will later be removed or

replaced when the functionality of the Unified Review Banner is fleshed

out further.
Currently, the Unified Review Banner is only shown when the Unified

Banner feature flag is enabled, by appending the following code to 

settings_local.py:
ENABLED_FEATURES = {
    'reviews.unified_banner': True
}

Added new JS tests to test functionality added. All JS tests pass.
Manual test cases:

1. Review request draft by modifying description, testing done, 

information -> banner should turn green

2. Review request draft by adding a file (through drag-and-drop and 

Update > Add File) -> banner should turn green

3. Discard review request draft with button on unified banner -> banner

should turn brown, review request discarded

4. New review by adding comment in diff viewer -> banner should turn 

green

5. Discard review with button on unified banner -> banner should turn 

brown, review discarded

6. New review with header, general comment in review dialog -> banner 

should turn green

7. Discard review in review dialog -> banner should turn brown, review 

discarded

8. Review reply draft by adding review replies on one review -> banner 

should turn green

9. Remove all text of review reply draft (empty string) -> review reply 

should be removed, banner should turn brown

10. Review reply draft by adding review replies on multiple/all reviews 

-> banner should turn green

11. Discard review reply drafts with button on unified banner -> banner 

should turn brown, all review replies discarded

12. New review/review request and review replies added -> banner should 

turn green

13. Discard with button on unified banner -> banner should turn brown, 

review and review replies discarded
*reload case (where on reload the correct banner state persists) tested

on all non-discard test cases