Add cloture rule

After some negative feedback on !33 (closed), I am trying again. This particular design came out of a discussion that arose at the end of today's quite-successful social event (thanks, @trac-Niki!). It requires a 2/3 majority to close off debate and is designed to work well in an asynchronous vote.

For clarity: though the general design came from a discussion, this text is by me alone.

This will require a 2/3 majority vote to pass, which translates to 9 Approvals.

Deadline: 1 week from today (10 Feb). This deadline means that this rule will not be adopted before 10 Feb.