Fixed disabling validators; fixed regressions

Now we remove disabled validators from `validators.remote` on client so we won't make requests to them. Fixes #468

Also fixes error when js is inserted after form and we don't have `ClientSideValidations.patterns`, and moves `root_path` detection to config since it fails to work with paths with params produced by default_url_options.

In fact it can be set from root_path, but that'd require more thorough sanitisation. Maybe next time.