(core) tweak embedding to play well with forking

Summary:
In the transition from a "pre-fork" to fork, when embedded,
the fork ends up being read-only and changes fail.  This commit
avoids applying the read-only default to forks.

If a user actually wants to specifically embed a fork as read-only,
they can still do so, by explicitly adding `/m/view`.

Test Plan: added test

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2723

app/common/gristUrls.ts

@@ -257,8 +257,8 @@ export function decodeUrl(gristConfig: Partial<GristLoadConfig>, location: Locat
   }
   if (sp.has('embed')) {
     const embed = state.params!.embed = isAffirmative(sp.get('embed'));
-    // Turn view mode on if no mode has been specified.
-    if (embed && !state.mode) { state.mode = 'view'; }
+    // Turn view mode on if no mode has been specified, and not a fork.
+    if (embed && !state.mode && !state.fork) { state.mode = 'view'; }
     // Turn on light style if no style has been specified.
     if (embed && !state.params!.style) { state.params!.style = 'light'; }
   }