(core) Treating API urls as external in cells

Summary:
Links for the API endpoints in a cell didn't work as they were interpreted as
internal routes. Now they are properly detected as external.

Test Plan: Added new test

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D4078

test/common/gristUrls.ts

@@ -45,6 +45,18 @@ describe('gristUrls', function() {
         {params: {themeName: 'GristDark'}},
       );
     });
+
+    it('should detect API URLs', function() {
+      assertUrlDecode(
+        'http://localhost/o/docs/api/docs',
+        {api: true},
+      );
+
+      assertUrlDecode(
+        'http://public.getgrist.com/api/docs',
+        {api: true},
+      );
+    });
   });
 
   describe('parseFirstUrlPart', function() {