Make the persistence method of hanging protocols configurable

- Implement ProtocolStore interface to allow persisting hanging protocols using different strategies
- Implement and set default strategy to persist hanging protocols in the MongoDB collection “HangingProtocols” in the application server

Packages/ohif-hanging-protocols/client/components/protocolEditor/protocolEditor.html

@@ -120,7 +120,6 @@
             <p class="lastSavedText">Last saved {{jsDateFromNow modifiedDate}}</p>
             {{/if}}
         </div>
-        <iframe id="download_iframe" style="display:none;"></iframe>
         {{/with}}
     </div>
 </template>