(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/ui/PagePanels.ts

@@ -63,6 +63,7 @@ export function pagePanels(page: PageContents) {
 
     cssMainPane(
       cssTopHeader(
+        testId('top-header'),
         (left.hideOpener ? null :
           cssPanelOpener('PanelRight', cssPanelOpener.cls('-open', left.panelOpen),
             testId('left-opener'),
@@ -144,6 +145,9 @@ export const cssLeftPane = styled(cssVBox, `
       display: none;
     }
   }
+  .interface-light & {
+    display: none;
+  }
 `);
 const cssMainPane = styled(cssVBox, `
   position: relative;
@@ -170,6 +174,9 @@ const cssRightPane = styled(cssVBox, `
       display: none;
     }
   }
+  .interface-light & {
+    display: none;
+  }
 `);
 const cssTopHeader = styled('div', `
   height: 48px;
@@ -184,6 +191,10 @@ const cssTopHeader = styled('div', `
       display: none;
     }
   }
+
+  .interface-light & {
+    display: none;
+  }
 `);
 const cssResizeFlexVHandle = styled(resizeFlexVHandle, `
   --resize-handle-color: ${colors.mediumGrey};