New: Manual import improvements (#683)

* New: Manual import improvements

 - Detect and merge import with files already in library.
 - Allow selection of album release from Manual Import modal.
 - Loading indicator while fetching updated decisions

* Disable release switching if user manually overrode release

frontend/src/Wanted/Missing/Missing.js

@@ -271,6 +271,7 @@ class Missing extends Component {
           <InteractiveImportModal
             isOpen={isInteractiveImportModalOpen}
             onModalClose={this.onInteractiveImportModalClose}
+            showReplaceExistingFiles={true}
           />
         </PageContentBodyConnector>
       </PageContent>