Edx-i18n-tools

Update travis file using new requirements.
Update tox file using new requirements.
Remove old requirement files.
Create requirements folder.
create pip_tools, base, test, travis and tox pip-tools requirements files.
Create Makefile and add upgrade command.
Include requirements files generated using upgrade command.
Update openedx.yml OEPs list.

- Added pyhton 3.8 support
- Removed support for django's versions below 2.2

Updated transifex to pull atleast 3% translated files instead of 10%.

1. Test that tag validation catches HTML added to translations.

2. When tag validation finds differences, output should be deterministic.