[Glitch] Allow multiple files upload through web UI, including drag & drop

Port 750c67660d to glitch-soc

app/javascript/flavours/glitch/features/composer/options/index.js

@@ -214,6 +214,7 @@ export default class ComposerOptions extends React.PureComponent {
           onChange={handleChangeFiles}
           ref={handleRefFileElement}
           type='file'
+          multiple
           {...hiddenComponent}
         />
         <Dropdown

app/javascript/flavours/glitch/features/ui/index.js

@@ -186,7 +186,7 @@ export default class UI extends React.Component {
     this.setState({ draggingOver: false });
     this.dragTargets = [];
 
-    if (e.dataTransfer && e.dataTransfer.files.length === 1) {
+    if (e.dataTransfer && e.dataTransfer.files.length >= 1) {
       this.props.dispatch(uploadCompose(e.dataTransfer.files));
     }
   }