Don't prevent users from saving an existing social application due to provider ID validation

[LMNTL]

… provider ID validation

Checklist

1. If you've added code that should be tested, add tests
2. If you've changed APIs, update (or create!) the documentation
3. Ensure the tests pass
4. Make sure that your code lints and that you've followed our coding style
5. Write a title and, if necessary, a description of your work suitable for publishing in our release notes
6. Mention any related issues in this repository (as #ISSUE) and in other repositories (as kobotoolbox/other#ISSUE)
7. Open an issue in the docs if there are UI/UX changes

Description

Fixed a bug that prevent updating Social Applications in the admin.

Notes

This problem was introduced by the recent django-allauth changes (kpi#4715); the check to see if the provider and provider_id fields were unique didn't account for social applications that were being edited, just new ones. So adding a Social Application would work fine, but editing an existing Social Application and clicking 'Save' would fail with a validation error.

[RuthShryock]

Looks good!