Add client validation for registration form

* also extract registration logic to users.js module

src/mutation_types.js

@@ -0,0 +1,7 @@
+export const SIGN_UP = {
+  SUCCESS: 'SIGN_UP_SUCCESS',
+  FAILURE: 'SIGN_UP_FAILURE',
+  PENDING: 'SIGN_UP_PENDING',
+  isPending: 'sign_up_pending',
+  errors: 'sign_up_errors'
+}