Put online rental history users on a TextIt follow-up campaign.

[toolness]

This fixes #906 and puts folks who filled out their rental history on a followup campaign.

The rental history campaign is configured via a new RAPIDPRO_FOLLOWUP_CAMPAIGN_RH setting, which is the comma-separated value of the campaign's group name and its field key (explanations for these can be found in #902 or by reading this PR).

New follow-up campaigns can be defined in the future with the RAPIDPRO_FOLLOWUP_CAMPAIGN_ prefix, so it won't be hard to trigger follow-up campaigns for LoC, HP Action, and future actions.

The new validate_rapidpro_settings management command ensures that all configured campaigns have valid group names and date field keys.

Users can also be manually put on follow-up campaigns via a new trigger_followup_campaign management command.

To do

• Do everything asynchronously (via Celery).
• Define an environment variable that specifies the group name/field key to use.
• Make it easy to validate that the group name/field key are valid.
• Actually put rental history users on the follow-up campaign.
• Add tests.