implement task recurrence templates

[jrheard]

my plan is: start by just implementing the ability to create and store "recurrence templates" in the api/cli/db. we don't actually need to do anything with them until later, so i can continue to put off thinking about the daemon.

here's the plan from my notes:

i think that the way to do it is to just have a RecurrenceTemplate model that's really similar to Task

but without these fields:
    mode

and with these additional fields:
    recurrence_period - how often we should make a task based off of this template
    due (as an optional duration/period as opposed to a date) - how many days in the future the task is due once it's been created

open questions:
    how do we know whether or not it's time to make a new instance of a recurring task?
        idea:
            * figure out what period we're in (eg 7/21-7/28)
            * see if a task exists that:
                * has this recurrence template as its .recurrence_template_id
                * was created during this period

Task and NewTask will gain fields:
    recurrence_template_id nullable foreign key

• add recurrence_template table
• implement creating recurrence templates
• implement listing recurrence templates
• implement recur info
• implement recur modify
• figure out what deletion will look like - do we do a hard delete, or do we add a mode column?