(core) Improve snapshot listing, and add compare snapshot links

Summary:
- When viewing a snapshot, list all snapshots for a doc, highlighting the current one.
- Include links in per-snapshot menu to compare-to-current and compare-to-previous.
- Compare links include "beta" tags.
- Set order of comparison to have older on the left, and newer on the right.

Test Plan: Moved out DocHistory test from Snapshots, and added some test cases.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2681

app/client/ui2018/menus.ts

@@ -255,6 +255,10 @@ export function menuItemCmd(cmd: Command, label: string, ...args: DomElementArg[
   );
 }
 
+export function menuAnnotate(text: string) {
+  return cssAnnotateMenuItem('Beta');
+}
+
 export const menuDivider = styled(weasel.cssMenuDivider, `
   margin: 8px 0;
 `);
@@ -374,3 +378,17 @@ const cssCmdKey = styled('span', `
   color: ${colors.slate};
   margin-right: -12px;
 `);
+
+const cssAnnotateMenuItem = styled('span', `
+  color: ${colors.lightGreen};
+  text-transform: uppercase;
+  font-size: 8px;
+  vertical-align: super;
+  margin-top: -4px;
+  margin-left: 4px;
+  font-weight: bold;
+
+  .${weasel.cssMenuItem.className}-sel > & {
+    color: white;
+  }
+`);