Improve edit locking mechanism

Problem/Motivation

Pagedesigner supports a custom locking mechanism while an editor is working on the pagedesigner content. Currently, the lock is created on entering edit mode and deleted when the user navigates away or the tab is closed. An undeleted lock is valid for 15 minutes. The lock is scoped to one tab/window to prevent the same user editing the same page in different tabs/windows.

Proposed resolution

Rework the locking mechanism to reflect current editing state better.

Remaining tasks

We will alter the locking mechanism as follows:

• Check and renew locks on edit action.
• Make sure locks are deleted on reload.
• Ignore locks older than 3 minutes (no editing activity).
• Add a permission so roles with that permission can delete locks at "/admin/config/pagedesigner/locks".