Implement form and page processing for triggered emails (Reminders)

This issue implements the schema from CRM-8150.

Please implement create/edit email triggers as well as a page to browse email triggers.

Draft implementation notes
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Yash / Kurund - pls review and modify or discuss w/ me after you've reviewed this draft. dgg

1. Tentative "name" for this functionality is "Schedule Reminders".

2. Menu access to the list of reminders is via Administer > Configure > Schedule Reminders.

3. Selector could include these columns:

• Title
• Reminder For (this is the "entity" and "type" shown on schedule.png - e.g "Activity - Evaluation")
• When (e.g. "2 hours after Activity Date Time)
• While (e.g. "Scheduled")
• Repeat ("Yes" or "No" - probably don't need repeat details in selector)
• Active? (e.g. reminders can be disabled w/o deleting them)

4. Notes on form mockup shown in scheduled.png (attached)

• we might want to freeze the fields for now where we are only supported a single option including:
  Entity (we're only supporting Activity)
  When-Date Field (I think we're only supporting activity_date_time for this iteration)
• not sure if we need the [x] Send Email checkbox?? I guess the idea is you could create a reminder that doesn't do anything except invoke the reminder_triggered hook - and for those you would un-check this field ?? Tim O - comments please
• if a template is used, and modified in the form - display the Update Template, Save as New Template optiions
• token insert widget needed for title and body fields
• default email editing section to HTML open / TEXT closed as we do in other similar forms