(core) switch to newer download endpoint in client

Summary:
 * Fix old download endpoint to correctly pass org info in redirect.
 * Switch to use newer download endpoint in client.

Old endpoint not removed. I started doing that, but it is used in copying, and it struck me that I'm not sure what should happen when copying from a site document to "Personal" - should it be the Personal that is associated with docs.getgrist.com currently, of should it be the Personal that is associated with the email of the user on whatever-site-we-are-on.getgrist.com. So leaving that as separate work.

Test Plan: updated tests

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2639

app/server/lib/requestUtils.ts

@@ -49,11 +49,20 @@ export function adaptServerUrl(url: URL, req: RequestWithOrg): void {
 
 /**
  * If org is not encoded in domain, prefix it to path - otherwise leave path unchanged.
+ * The domain is extracted from the request, so this method is only useful for constructing
+ * urls that stay within that domain.
  */
 export function addOrgToPathIfNeeded(req: RequestWithOrg, path: string): string {
   return (isOrgInPathOnly(req.hostname) && req.org) ? `/o/${req.org}${path}` : path;
 }
 
+/**
+ * If org is known, prefix it to path unconditionally.
+ */
+export function addOrgToPath(req: RequestWithOrg, path: string): string {
+  return req.org ? `/o/${req.org}${path}` : path;
+}
+
 /**
  * Returns true for requests from permitted origins.  For such requests, an
  * "Access-Control-Allow-Origin" header is added to the response.  Vary: Origin