(core) When saving copies, allow saving to another org; update menus for making and saving copies.

Summary:
- Implemented selecting an org in some cases when using Save-Copy dialog.
- Unified previous 'Save Copy' menu into an enhanced "Share" menu.
- Renamed ExportMenu to ShareMenu, collect related code into it, and design the share button.
- Introduced trunkAccess property for forks, to know whether "Replace Original" is available.
- Simplified handling of fork() result, now that all code has been upgraded.
- Replaced 'Copy as Template' menu items with a checkbox in the Save-Copy dialog
- Removed copy links for examples in the DocMenu (to simplify, since not part of updated design)
- Updated the UI of the copying dialog.

Test Plan: Updated affected tests, added new test cases for copying when other orgs are a choice or not.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2561

app/common/ActiveDocAPI.ts

@@ -220,7 +220,6 @@ export interface ActiveDocAPI {
 
   /**
    * Prepare a fork of the document, and return the id(s) of the fork.
-   * TODO: remove string option here, it is present to ease transition.
    */
-  fork(): Promise<string | ForkResult>;
+  fork(): Promise<ForkResult>;
 }

app/common/UserAPI.ts

@@ -99,6 +99,7 @@ export interface Document extends DocumentProperties {
   id: string;
   workspace: Workspace;
   access: roles.Role;
+  trunkAccess?: roles.Role|null;
 }
 
 export interface PermissionDelta {