Rebuild workspace menu trees automatically

Problem/Motivation

Workspace menu trees going out of sync is problematic as it’s confusing for content editors. Menu trees should be rebuilt automatically, given that it does seem a costly operation.

Proposed resolution

Add a wse_menu_tree_needs_rebuild key/value entry, which should be an array keyed by workspace ID, and with TRUE/FALSE values. Whenever the menu tree changes in Live, we need to update that k/v entry and set it to TRUE for all workspaces. Then, in a request subscriber, after checking that a workspace is active, check whether we need to rebuild the menu tree for that workspace by looking at the k/v entry, and perform the rebuild if needed.