Allow multiple files upload through web UI, including drag & drop (#9856)

* Allow drag and drop uploads of multiple files to compose

* Calculate aggregate upload progress for single action

* Allow multiple uploads to compose through traditional input, consolidate update file limit logic, provide file limit feedback

app/javascript/mastodon/features/compose/components/upload_button.js

@@ -63,7 +63,7 @@ class UploadButton extends ImmutablePureComponent {
             key={resetFileKey}
             ref={this.setRef}
             type='file'
-            multiple={false}
+            multiple
             accept={acceptContentTypes.toArray().join(',')}
             onChange={this.handleChange}
             disabled={disabled}