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/locales/defaultMessages.json

@@ -12,6 +12,15 @@
     ],
     "path": "app/javascript/mastodon/actions/alerts.json"
   },
+  {
+    "descriptors": [
+      {
+        "defaultMessage": "File upload limit exceeded.",
+        "id": "upload_error.limit"
+      }
+    ],
+    "path": "app/javascript/mastodon/actions/compose.json"
+  },
   {
     "descriptors": [
       {
@@ -2285,4 +2294,4 @@
     ],
     "path": "app/javascript/mastodon/features/video/index.json"
   }
-]
+]