Framework for scheduled messages and callbacks

[Laogeodritt]

Create a framework in the core for:

• Scheduling an event for later, either using a datetime or an interval of time
• Managing those events and the event loop tasks behind them
• Allow cancelling those events (including at bot shutdown)
• Sending a Discord message and/or calling a coroutine/future

This should effectively just be a little syntactic sugar for the

async def scheduled_task(self):
    await asyncio.sleep(get_time_in_seconds())
    do_something_else()

paradigm that's used everywhere, along with the management of starting and cancelling the task (maybe allow specifying a unique identifier, or having some form of identifier + unique/non-unique option?).

Existing cogs to adapt:

• reminders
• sprints, sprint weekly digest
• userstat reports/anonymisation
• spotlight reminders
• spotlight #167
• tempbans in modtools
• others?

Related issues: #167 #127 #96

[Laogeodritt]

Should look further into the Task API and capabilities, e.g.: http://masnun.com/2015/11/20/python-asyncio-future-task-and-the-event-loop.html

[Laogeodritt]

Scheduler framework added in b04001f (+ subsequent)

[Laogeodritt]

415ce37 - reminders updated

[Laogeodritt]

af960c8 - sprint