Adds val trancripts in outcome of get_transcripts_info and rest of flow remains the same and also add fallback  to edx-val rtanscripts for mobile accessible video endpoints.

Purpose of this, is to serve transcripts from edx-val if the requested transcripts are not found in the contentstore i.e. that's where all the transcripts are living to date.

EDUCATOR-1216

EDU-1092

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