Add missing image model spec. Add shared specs to check image validations at any imageable model

spec/models/image_spec.rb

@@ -0,0 +1,8 @@
+require 'rails_helper'
+
+describe Image do
+
+  it_behaves_like "image validations", "budget_investment_image"
+  it_behaves_like "image validations", "proposal_image"
+
+end