(core) Enhancements to the Public Access UI.

Summary:
- Show a 'Copy Link' button in UserManager.
- Add icons for Copy (to copy link), and also for Video (to open video
  tutorials in later onboarding changes)
- Add to UserManager a 'Public Access' member-like line for greater visibility
  and to allow changing role.
- In main document page, add a "public access" icon.
- On saving UserManager, re-fetch DocInfo to update "public access" icon.

Test Plan: TBD

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2617

app/client/models/gristUrlState.ts

@@ -44,6 +44,9 @@ export function docUrl(doc: Document): IGristUrlState {
 // Returns the home page for the current org.
 export function getMainOrgUrl(): string { return urlState().makeUrl({}); }
 
+// When on a document URL, returns the URL with just the doc ID, omitting other bits (like page).
+export function getCurrentDocUrl(): string { return urlState().makeUrl({docPage: undefined}); }
+
 // Get url for the login page, which will then redirect to nextUrl (current page by default).
 export function getLoginUrl(nextUrl: string = _getCurrentUrl()): string {
   return _getLoginLogoutUrl('login', nextUrl);