Adding Django 2.x support

[FinalAngel]

WIP

• Needs release of django-tablib
• Needs release of django-emailit

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@2932c85). Click here to learn what that means.
The diff coverage is 52.72%.

@@            Coverage Diff            @@
##             master     #171   +/-   ##
=========================================
  Coverage          ?   53.68%           
=========================================
  Files             ?       35           
  Lines             ?      786           
  Branches          ?       75           
=========================================
  Hits              ?      422           
  Misses            ?      362           
  Partials          ?        2

Impacted Files	Coverage Δ
aldryn_forms/action_backends.py	80.95% <ø> (ø)
aldryn_forms/migrations/0009_auto_20171218_1049.py	100% <ø> (ø)
aldryn_forms/migrations/0010_auto_20171220_1733.py	63.15% <ø> (ø)
aldryn_forms/urls.py	100% <ø> (ø)
...ldryn_forms/contrib/email_notifications/helpers.py	56.25% <ø> (ø)
aldryn_forms/signals.py	100% <ø> (ø)
aldryn_forms/validators.py	57.89% <ø> (ø)
aldryn_forms/contrib/email_notifications/models.py	45.87% <100%> (ø)
aldryn_forms/admin/forms.py	40.74% <100%> (ø)
...otifications/migrations/0004_auto_20190104_1242.py	100% <100%> (ø)
... and 14 more

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