simplify jsp layout management

src/main/webapp/WEB-INF/jsp/exception.jsp

...ain/webapp/WEB-INF/jsp/owners/createOrUpdateOwnerForm.jsp

src/main/webapp/WEB-INF/jsp/owners/findOwners.jsp

src/main/webapp/WEB-INF/jsp/owners/ownerDetails.jsp

src/main/webapp/WEB-INF/jsp/owners/ownersList.jsp

src/main/webapp/WEB-INF/jsp/pets/createOrUpdatePetForm.jsp

src/main/webapp/WEB-INF/jsp/pets/createOrUpdateVisitForm.jsp

src/main/webapp/WEB-INF/jsp/vets/vetList.jsp

src/main/webapp/WEB-INF/jsp/welcome.jsp

src/main/webapp/WEB-INF/tags/layout.tag

src/main/webapp/WEB-INF/tags/menu.tag

src/main/webapp/WEB-INF/tags/menuItem.tag