Every integration defined its own ConditionsField with its own label,

providing a ReviewRequestConditionChoices class. Paired with design

flaws in Djblets and Review Board with extensible conditions, this meant

that there was no way to show any condition choices registered with any

central registry.
Review Board is gaining a centralized ReviewRequestConditionsField

and a central registry that reflects any choices provided by extensions.

This change makes use of that, providing a compatibility class for older

versions of Review Board that behaves as before.
All ConditionsField instances have been updated to utilize the new

standard field.
The Travis-CI support had a special requirement where only GitHub

repositories were shown. To make this work, we now provide a specialized

registry that's instantiated only when the field is ready to be used. It

wraps the main registry and replaces repository-related condition choices

with ones that are limited to GitHub repositories.

Tested with Review Board 7.0.2 and 7.1 (with the new registry support

for condition choices). Verified that on 7.1, extension-provided choices

appeared in the list and were matched.
Tested that the Travis-CI support only included GitHub repositories.