Upgrade edx-search to use Elasticsearch 7

This version is a major upgrade to bump the version of ES used by edx-search from 1.5 to 7.8. This change is not backwards compatible.

Fixed python 3.8 travis worker

Merge pull request #100 from edx/iamsobanjaved/py38

Added python 3.8 travis worker

Added python 3.8 support

• Added python 3.8 support
• Removed Django support for versions prior to 2.2

Django 2.2 compatibility

Merge pull request #95 from edx/timmc/BOM-1481-unpin-django

Relax Django constraint for upgrade to 2.2

Revert out more backwards-incompatible search logic

Merge pull request #93 from edx/diana/revert=sort

Revert out sorting changes that cause edx-platform tests to fail.

Revert discovery sorting options

We are reverting this commit to unblock updates on edx-platform.

Django 2.0, 2.1, and 2.2 Compatibility

Django 2.0, 2.1, and 2.2 Compatibility

Allow discovery search results to be sorted

Merge pull request #72 from Stanford-Online/discovery-search-sort

Allow discovery search results to be sorted

Making repo oep7 and oep 18 compliant

Merge pull request #79 from edx/msingh/version_number

PROD-384: Updated version number to 1.2.3

Fix missing search excerpts when query has quotes

Merge pull request #51 from open-craft/kshitij/fix-empty-search-quotes

Fix missing search excerpts when query has quotes