Add model for per-user-settings

[jnm]

allowing us to selectively enable Intercom. Closes #1804. Note that existing INTERCOM_APP_IDs must be migrated from

kpi/kobo/settings/base.py

Line 334 in 0c49d3a

INTERCOM_APP_ID = os.environ.get('INTERCOM_APP_ID')

to hub.models.PerUserSetting (available in the Django admin interface).

Post-merge

The following PRs must accompany this one:

• Remove obsolete INTERCOM_APP_ID env. var. kobo-docker#241
• Remove obsolete INTERCOM_APP_ID kobo-install#31

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 your code lints and you followed our coding style
5. [ ] If this is a big feature, make sure to prefix the title with Feature: and add a thorough description for non-dev folk