When we first wrote the support for condition choices for integrations,

we intended for the framework to be extensible, allowing extensions to

produce new condition choices that users could select from. However,

this didn't work in practice, since we lacked any kind of central

registry for condition choices that extensions could hook into.
This change introduces a new central review_request_condition_choices

registry, along with a new ReviewRequestConditionChoicesHook that

wraps it.
A central ReviewRequestConditionsField can be used whenever a

ConditionsField is meant to be used with review_request_condition_choices,

enabling custom condition choices from any extension. rbintegrations 4.1

will be updated to use this.
Documentation has been written to help extension authors make use of the

new hook. It only touches upon the basics of creating new choices, and

doesn't touch upon custom operators, but it should be enough to get

people going.

Unit tests passed.
Tested the new central registry with in-progress code in rbintegrations

and Power Pack.
Built the docs. Checked the example code against the unit tests, and

checked for bad links and other errors.