This is the first step of the larger project of creating the Unified

Review Banner Base Shell. Adding this feature flag will allow us to

toggle between the current banner experience and the new Unified Review

Banner experience to allow development on the Unified Banner to continue

without breaking the current experience.
Out of scope for this patch: using the feature flag to toggle between 

the two experiences (will come in the next patch).

Ran all JS Unit tests (all pass)
Set the value in settings_local.py as 

ENABLED_FEATURES = {
    'reviews.unified_banner': True/False
}

and tested output of RB.EnabledFeatures.unifiedBanner in 

ReviewablePage.es6.js file (where the Unified Banner will be 

instantiated)