(core) support ?embed=true and &style=light for a clean embed experience

Summary:
This adds query parameters useful for tailoring the Grist experience, with an eye to embedding.

Setting `style=light` removes side and top bars, as a first pass at a focused view of a single document page (this would benefit from refining).

Setting `embed=true` has no significant effect just yet other than it restricts document access to viewer at most (this can be overridden by specifying `/m/default`).

Test Plan: added tests

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2585

app/common/gutil.ts

@@ -780,3 +780,12 @@ export async function isLongerThan(promise: Promise<any>, timeoutMsec: number):
   ]);
   return isPending;
 }
+
+/**
+ * Returns true if the parameter, when rendered as a string, matches
+ * 1, on, or true (case insensitively).  Useful for processing query
+ * parameters that may have been manually set.
+ */
+export function isAffirmative(parameter: any): boolean {
+  return ['1', 'on', 'true', 'yes'].includes(String(parameter).toLowerCase());
+}