Updating the django-rate-limit requirement.
updated the django-ratelimit to use unreleased version that supports Django 2.2

Django-auth-toolkit update

Pinned the latest commit from github to get Django2.2 support in testing.
Pinned the zipp<2.0.0 to get Python<3.5 support too.

* Constraining edx-rest-api-client

* Decreased pin on edx-rest-api-client from <3.0.0 to <2.0.0

Unpin several more outdated dependencies whose changelogs don't contain any significant backwards incompatible changes. Also add "moto" to the list of packages to uninstall from existing environments, since it requires a jsondiff version that clashes with the one we now use (triggering a harmless but distracting warning on dependency updates).

We can potentially stop using path.py/path altogether by switching to pathlib in the Python 3 standard library, but that merits a separate PR of its own.

Also, note that I'm not actually unpinning freezegun; different PRs restricted it in both constraints.txt and test.in, I'm just removing the latter redundant constraint.

Added the link to latest master commit of GitHub for Django 2.2 support.

Removing the moto.

django-appconf bump version using commit hash for django2.2

This is step 3 of DE-1824.

Also update:
* six 1.13.0 => 1.14.0
* edx-django-utils 2.0.2 => 2.0.3
* skip test that checks all model changes have migrations.  (We defer
  this to step 4.)

Removed the version constraint for django-mysql
Also pinned the freezegun version to 0.3.12 as the latest version is causing failures.

Bump django-filter.
Rename the name to field-name.

Wiki version bump.

bumping enterprise version to 2.0.45

Updating wiki.