Ensure that a generated form's page seeds are idempotent.

Fixes #2673
Move url variable out to a less confusing placement.
When generating a form, only depend on a semantic version of refinerycms-settings.

This means we will get ~> 2.1.0 instead of ~> 2.1.2 so we won't
have to push a new refinerycms-settings gem for each version.