(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/ui2018/cssVars.ts

@@ -6,6 +6,7 @@
  *  https://css-tricks.com/snippets/css/system-font-stack/
  *
  */
+import {urlState} from 'app/client/models/gristUrlState';
 import {getTheme, ProductFlavor} from 'app/client/ui/CustomThemes';
 import {dom, makeTestId, styled, TestId} from 'grainjs';
 import values = require('lodash/values');
@@ -164,4 +165,6 @@ export function attachCssRootVars(productFlavor: ProductFlavor, varsOnly: boolea
   if (theme.bodyClassName) {
     document.body.classList.add(theme.bodyClassName);
   }
+  const interfaceStyle = urlState().state.get().params?.style || 'full';
+  document.body.classList.add(`interface-${interfaceStyle}`);
 }