(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/client/lib/log.ts

@@ -0,0 +1,15 @@
+/**
+ * Client-side debug logging.
+ * At the moment this simply logs to the browser console, but it's still useful to have dedicated
+ * methods to allow collecting them in the future, or silencing them in production or in mocha
+ * tests.
+ */
+
+export type LogMethod = (message: string, ...args: any[]) => void;
+
+// tslint:disable:no-console
+export const debug: LogMethod = console.debug.bind(console);
+export const info: LogMethod = console.info.bind(console);
+export const log: LogMethod = console.log.bind(console);
+export const warn: LogMethod = console.warn.bind(console);
+export const error: LogMethod = console.error.bind(console);