Implement minimum and maximum timespans to protect against DoS

[svierne]

Observed behaviour

With the current implementation, the bot can be DoSed by setting up a recurring reminder as follows:
!remindroom every 1 second; now; spam

Proposed fix

Implement config variables for minimum and maximum timespans, e.g.

reminders:
    timespan_min: "1 minute"
    timespan_max: "1 year"

For reminders above and below the timespan limit, the bot should respond with an error.

[HarHarLinks]

There are also already rate limits for the different endpoints in the Client-Server API to prevent similar spam. I think it's good to act like a "good citizen" trying to prevent "spam" at the source.

I wonder if something in nio already takes care of rate limits blocking messages from being sent and retrying. There might not.

I'm assuming these are supposed to apply to the intervals of repeating reminders. What would be the purpose of the upper limit?

Note to potential implementers: remember cronjobs as well.

[svierne]

The upper limit will prevent, for example, reminders that are created just for fun, reminding you of the new century 2100 or something.