Support "create" access for webform submissions.

Currently, weborm_entity_submission_access() always returns FALSE for 'create' ops. This is because there's no clean way to determine access to submitting a webform (the logic is tangled into webform_node_view()) -- see #1794960: Put the logic for checking if a user can submit webforms into a function.

However, since there's been no movement on that issue, can we at least implement the suggested workaround. Otherwise, it's impossible to create a webform submission via the entity api.