Retrieve different data fields from Discovery

* Includes custom manager.
* Includes percent validation.
* Includes useful indices.
* Subclasses TimeStampedModel

OC-3086

Implement a course dates fragment as a web view, currently we are using
the course info page to get the course dates section by parsing the
whole html page.

LEARNER-2769

This avoids a fatal error in Django 1.9+ in most scenarios, where this
code (before this commit) will no longer result in an implicit __in
query.  This commit should be a no-op functionally for Django 1.8.

More info: https://code.djangoproject.com/ticket/25284

PLAT-1525

LEARNER-142

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