the main point of this change is to allow a staff user to specify submit_as=a-different-user when creating a review request so that user will be the one notified of reviews, etc.  this enables using a post-commit hook to auto-create reviews for all or part of a repository.  there are some side concerns related to this:
 
 * basedir must be allowed to be empty when creating diffs against a svn repository as opposed to a working copy.  that is what the diffviewer change does; for the web interface the functionality is identical, but the json manually rips out the basedir when necessary, allowing a svn repo diff to be treated as "absolute."
 * added is_visible_to and is_mutable_by methods to the Request models to encapsulate the notion that "staff can always view and modify requests"
 * moved sending of mail into the models to avoid duplication of code in json and view
 * json new_diff includes traceback when something unexpected goes wrong
 * add get_changeset to svn to allow update_from_changenum for that scm