uploads: fix broken tests.

* Fix broken upload tests.
* Fix uploads to return an error if both a file and a source are given
  at the same time, or if neither are given. Also fix the error message
  in this case so that it doesn't include "base" at the start of the string.
* Fix uploads to percent-encode any Unicode characters in the source URL.
* Add a max filesize validation to media assets.

app/components/file_upload_component/file_upload_component.js

@@ -110,7 +110,13 @@ export default class FileUploadComponent {
   async onError(e) {
     let errors = e.originalEvent.detail[0].errors;
     let message = Object.keys(errors).map(attribute => {
-      return errors[attribute].map(error => `${capitalize(attribute)} ${error}`);
+      return errors[attribute].map(error => {
+        if (attribute === "base") {
+          return `${error}`;
+        } else {
+          return `${capitalize(attribute)} ${error}`;
+        }
+      });
     }).join("; ");
 
     Utility.error(message);