Convert bbp_redirect_canonical into its own sub-action

[johnjamesjacoby]

Currently, bbp_redirect_canonical() is hooked into redirect_canonical and encapsulates a few different cases when redirections should be prevented.

As more of these cases are introduced, it would be nice for them to be hooked in more atomically, so that they can be customized and improved individually.

(I'm researching a potential security concern that was reported to the bbPress team via HackerOne, and it revealed some potential to improve this API before hand.)

[johnjamesjacoby]

In 7348:

Core - Canonical: Convert bbp_redirect_canonical() into a sub-action.

This change abstracts the existing internal canonical redirect logic into new dedicated functions so that they can be individually improved or customized in the future.

Pagination and Editing theme-side require intercepting and preventing the core redirection when requesting certain URLs, and there are likely to be new conditions in the future.

In trunk, for 2.7.

Fixes #3648.