Reminders bot

[pabzm]

This adds two bots to this repo:

1. Automatically comment on every PR that is stale for two weeks.
2. Allow to manually specify when someone shall be reminded of something.

The latter's usage is /remind [who] [what] [when].

Examples:

/remind me to deploy on Oct 10
/remind me next Monday to review the requirements
/remind me that the specs on the rotary girder need checked in 6 months
/remind @<username> to fix this issue tomorrow

[pabzm]

@alecpl FYI. If they get annoying we can remove them, but maybe they can help

[mvorisek]

@pabzm please remove this bot, it is more than annoying.

[pabzm]

I agree that in the current situation and state of this project, the bot is more annoying than helping → #9726