Django-recaptcha

- Added testing for Django 2.2 (no code changes needed).

- Moved field based Google dev key check to an app ready registered
security check.

- Bugfix: Remove extra div in widget_v3 template

- ReCAPTCHA v3 support added.
- Remove all mention of the V1 reCAPTCHA endpoint.
- Refactor client, fields and widgets code.
- Added widgets for each type of reCAPTCHA: `V2 Checkbox`,
`V2 Invisible`, `V3`
- Remove the need for the widget template to be selected based on
certain settings values, each widget has its own template.
- Introduced a large number of new unit tests, update tests to make
use of tox venvs.
- Regenerated po and mo files.

- Added testing for Django 2.1 (no code changes needed).
- Update the unit tests to no longer make use of reCAPTCHA v1.
- Added deprecation warnings for reCAPTCHA v1 support.
- Remove the need for RECAPTCHA_TESTING environment variable during
unit testing.
- Added Invisible reCAPTCHA V2 support.

- Dropped support for Django \< 1.11.
- Added testing for Django 2.0 (no code changes needed).