(core) Fix owner view access to snapshots

Summary:
Owners weren't able to access snapshots if access rules
that denied access to non-owners existed. The backend
was lowering snapshot document access to "viewers" as
part of implementing read-only behavior; this is now done
in the client, with document access for snapshots now
accurately reflecting the user's trunk access.

Additionally, sandboxes are no longer created for snapshots,
and background intervals aren't started for snapshots.

Test Plan: Browser test.

Reviewers: jarek, paulfitz

Reviewed By: jarek, paulfitz

Differential Revision: https://phab.getgrist.com/D3849

app/client/ui/ShareMenu.ts

@@ -36,7 +36,7 @@ export function buildShareMenuButton(pageModel: DocPageModel): DomContents {
   return dom.maybe(pageModel.currentDoc, (doc) => {
     const appModel = pageModel.appModel;
     const saveCopy = () => makeCopy(doc, appModel, t("Save Document")).catch(reportError);
-    if (doc.idParts.snapshotId) {
+    if (doc.isSnapshot) {
       const backToCurrent = () => urlState().pushUrl({doc: buildOriginalUrlId(doc.id, true)});
       return shareButton(t("Back to Current"), () => [
         menuManageUsers(doc, pageModel),

app/client/ui/TopBar.ts

@@ -86,7 +86,7 @@ export function createTopBarDoc(owner: MultiHolder, appModel: AppModel, pageMode
           isRecoveryMode: pageModel.isRecoveryMode,
           isTutorialFork: pageModel.isTutorialFork,
           isFiddle: Computed.create(owner, (use) => use(pageModel.isPrefork)),
-          isSnapshot: Computed.create(owner, doc, (use, _doc) => Boolean(_doc && _doc.idParts.snapshotId)),
+          isSnapshot: pageModel.isSnapshot,
           isPublic: Computed.create(owner, doc, (use, _doc) => Boolean(_doc && _doc.public)),
         })
       )