Replacing jsonfield2 with jsonfield as former is merged back into latter one.

BOM-2447

This moves the CourseEditLTIFieldsEnabledFlag from edx-platform to this repo. This allows using the flag from both lms and studio.

* Revert "chore: Replace jsonfield2 with jsonfield"

This reverts commit 2fd330f8.

* fix: only kept jsonfield related revert changes and rest are removed

* Add initial version of LineItem Implementation

Signed-off-by: Giovanni Cimolin da Silva <giovannicimolin@gmail.com>

* Adding tests

* Add LTI tests and fix quality isses

* Quality nit

* Add missing requirement, upgrade

* Squash migrations

* Fix merge conflicts

* Addressing review comments

This change moves the LTI configuration retrieval to
a Django model that will LTI configuration.
The current model proxies back the to blocks to keep
backwards compatibility.

See reasoning for change at:
https://github.com/edx/xblock-lti-consumer/blob/master/docs/decisions/0001-lti-extensions-plugin.rst

k# Please enter the commit message for your changes. Lines starting

Remove constraints.
Add classifiers.
Update setup.py install_requirements definition.
Update travis file using new requirements.
Create requirements folder.
create pip_tools, base, test and travis requirements files.
Remove old requirement files.
Create Makefile and add upgrade command.
Include requirements files generated using upgrade command.
Update openedx.yml OEPs list.