[Sendportal] Extend Drip campaign functionality with conditional triggers and actions

Extend [Drip campaign functionality](https://github.com/mettle/sendportal/issues/89) to allow mixing-and-matching any of the following triggers and actions. ## **Conditional drip campaigns, based on triggers:** - opening an email - clicking on a specific link in an email - opening a specific email but not clicking on a specific link (within configurable timeframe: X hours, X days) - opening a specific email but not clicking on any link (within configurable timeframe: X hours, X days) - not opening the email (within configurable timeframe: X hours, X days) ## **Take different actions:** - Remove the user from an existing Drip campaign (so as not to spam them) - Add the user to a different Drip campaign (to give them a different offer) - Send the user a one-time email - Tag the subscriber / user