(core) Add support for auto-copying docs on signup

Summary:
The new "copyDoc" query parameter on the login page sets a short-lived cookie, which is
then read when welcoming a new user to copy that document to their Home workspace, and
redirect to it. Currently, only templates and bare forks set this parameter.

A new API endpoint for copying a document to a workspace was also added.

Test Plan: Browser tests.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3992

app/server/lib/requestUtils.ts

@@ -300,6 +300,17 @@ export function integerParam(p: any, name: string): number {
   throw new ApiError(`${name} parameter should be an integer: ${p}`, 400);
 }
 
+export function optBooleanParam(p: any, name: string): boolean|undefined {
+  if (p === undefined) { return p; }
+
+  return booleanParam(p, name);
+}
+
+export function booleanParam(p: any, name: string): boolean {
+  if (typeof p === 'boolean') { return p; }
+  throw new ApiError(`${name} parameter should be a boolean: ${p}`, 400);
+}
+
 export function optJsonParam(p: any, defaultValue: any): any {
   if (typeof p !== 'string') { return defaultValue; }
   return gutil.safeJsonParse(p, defaultValue);