This adds request caching to the following places:
* course expiration wrapper (displayed in Units)
* offer banner generation (displayed in Units)
* get_enrollment
* user_by_anonymous_id
* youtube_disabled_for_course

On a sample course with edx-val enabled, this reduced the queries
for a large sequence from 450 to 155.

This will remove imports from __future__ that are no longer needed.

https://docs.python.org/3.5/library/2to3.html#2to3fixer-future

This helps out the Verified Only case where the user
is not first enrolled as an audit learner.

REV-980

This reverts commit 059b6197.

REV-980.
* moved get_first_purchase_offer_banner_fragment to Discounts utils.py
* added banner wrapper method
* updated banner with time limit 

* REVMI-354 Add enroll_in_course permission

* Fix test

Currently, there are two purchase calls are occurring for each audit
purchase on sail-thru.To avoid it,code is updated so that a single
call will occur for each purchase.

LEARNER-5257

Django 1.10 deprecation fix for Hackathon XIX
Addresses PLAT-1397

We no longer need the drop_existing_session flag on IdP configurations
because dropping the existing session should actually be the only
behavior for certain view in the edx-enterprise code.

LEARNER-2412

LEARNER-2546

This reverts commit d940bbfd.