Enterprise- Version bumped to 0.51.5

Fix frontend email regex for login page.

Currently we are using Django 1.4 regex for email validation which
does not allow custom domains (e.g. eclo.solutions), fixed it with
latest Django 1.11 regex.

LEARNER-2828

YONK-776: added thread_followed and thread_unfollowed signals

Add edx-private and sandbox req files to tox env.

Course Update emails (initial)

Delete old recurring_nudge_schedules_for_hour task

Merge release back to master

address coursegraph issue where components had the same block id but …

Adding spanish certificate template partials learner-1996 

Remove all usage of the django-simple-history module.

Rewrite program marketing page to use Bootstrap and change design to look more like other marketing design

to the Micromasters page

Fix get_enterprise_customer api cache key.

Upsell nudges for unverified users

LEARNER-2819: Fix caching of course_outline_root_block.

Add RTL support for Bootstrap CSS

ENT-704

Fixed typos in comment and config  message.

LEARNER-2819

Modified criteria for upselling. User must have a dynamic deadline to receive an email with an upsell button

Separate the date and link logic out of VerifiedUpgradeDeadlineBlock, so that it can be called directly with prefetched data for check_and_get_upgrade_link_and_date

Fixed problem with non-Latin characters in loncapa/python problems

Fix get_enterprise_customer api cache key.

ENT-704

LEARNER-1705