bot, rules: fix rate limiting rules without a rate limit

[Exirel]

Description

Fix #2627 by checking if the rate limit is actually set (equal or less than 0s).

I decided to make the at_time parameter mandatory, as it's all internal stuffs and it makes it clearer what time we are comparing it to. That's how I figured out that it was about the repeating call to the same rule for a single trigger: it would check against the same trigger object, but compare it to the latest call to now(). That's why the bug was visible on url rules, and it affected find rules as well.

Checklist

• [x] I have read CONTRIBUTING.md
• [x] I can and do license this contribution under the EFLv2
• [x] No issues are reported by make qa (runs make lint and make test)
• [x] I have tested the functionality of the things this change touches

[Exirel]

Good to squash

Done.