This converts the New Review Request page to a class-based view,

allowing us to more easily extend the functionality of the page down the

road while keeping the view maintainable (which will be helpful when we

eventually add file browsing). It also substantially improves load times

by reducing the work needed to load information on each repository.
Previously, we needed to instantiate an instance of each repository's

SCMTool in order to determine the capabilities of the SCMTool, as those

capabilities were hidden behind methods on the instance. This isn't true

anymore, except in the case of one method that would only be implemented

by older custom SCMTools.
We now look up the capability flags from the class, falling back to

creating an instance only if needed. This means we don't have to

initialize the SCMTool (possibly performing an HTTP request or logging

into some service) for any SCMTools we ship, speeding up load times.

Tested that the New Review Request page fully works as it did before,

and that the SCMTools aren't initialized when loading the page.