(core) add a deployment test for Import-from-URL, and fix underlying issue

Summary:
 * Adds a simple deployment test for the "Import from URL" button.
 * Makes server aware of plugin hostnames in the appropriate places.
 * Unrelated but convenient: allows following redirection when importing.

Test Plan:
Added tests. The `local_deployment` test works.  A modified
version of this works against `staging_deployment` (using a test url that
doesn't require redirection; also staging currently has a hot fix that can
hopefully be removed once the code fix included here is in).

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2556

app/server/lib/uploads.ts

@@ -346,7 +346,11 @@ export async function fetchURL(url: string, accessId: string|null): Promise<Uplo
  */
 async function _fetchURL(url: string, accessId: string|null, fileName: string,
                          headers?: {[key: string]: string}): Promise<UploadResult> {
-  const response: FetchResponse = await Deps.fetch(url, {headers});
+  const response: FetchResponse = await Deps.fetch(url, {
+    redirect: 'follow',
+    follow: 10,
+    headers
+  });
   await _checkForError(response);
   if (fileName === '') {
     const disposition = response.headers.get('content-disposition') || '';