Extract proposal new from shared nested imageable specs to system specs

Make `path`, `fill_resource_method_name`, `submit_button`, and
`imageable_success_notice` dynamic based on the factory.

Also adjusted the user. The proposals no longer require the user to be an
administrator but do require them to be a level 2 user.

Note that we are adding the Style/CaseLikeIf rubocop rule.

spec/system/proposals_spec.rb

@@ -1291,14 +1291,6 @@ describe "Proposals" do
 
   it_behaves_like "imageable", "proposal", "proposal_path", { id: "id" }
 
-  it_behaves_like "nested imageable",
-                  "proposal",
-                  "new_proposal_path",
-                  {},
-                  "imageable_fill_new_valid_proposal",
-                  "Create proposal",
-                  "Proposal created successfully"
-
   it_behaves_like "nested imageable",
                   "proposal",
                   "edit_proposal_path",