Facebook / Twitter Auth is broken

[tevko]

Expected behavior:
As a user, I should be able to authenticate where required.
As an individual or organization using Polis, I should be able to configure authentication.

Actual behavior:
Clicking on FB / Twitter auth buttons leads to a broken authentication flow

Additional context:
Related to: #253

** Solution **
Integrate cloud auth provider for https://pol.is.

Allow for deployment configuration that enables self configured cloud auth provider or use of internal username / password configuration

[tevko]

#1840 - step 1