Ability to schedule automated tasks for groups of Data Sources

[josh-chamberlain]

Based on Police-Data-Accessibility-Project/data-sources-app#22

At a base level, this is a flexible job scheduler which allows people to set rules for recurrence and an action.

Examples:

Recurrence

• on a set date
• every x days, weeks, months
• x hours / minutes after the Data Source's refresh rate (in the database)
• when a particular Scraper is updated

Selection

• select a group of Data Sources via saved search Police-Data-Accessibility-Project/planning#177

Example Actions:

• run a particular scraper on a Data Source
• create an archive of a Data Source to a local machine
• send an email notification (https://github.com/Police-Data-Accessibility-Project/data-sources-app/issues/26)
• send a webhook