Allow setting review assignment limit

[apiraino]

Part of #1753

With this patch we allow team members to set their own review capacity.

This will work by sending a message to the triagebot bot on Zulip. Usage:

• work set #: will set the number of max PRs assigned to me

This setting will only work for a subset of Github users that volunteered (see here).

An update the documentation on the forge will follow up.

(I cannot test it, don't have yet a testing infrastructure with also a Zulip instance)

r? @jackh726

thank you