[EW] Handle read-only users in EW correctly

[usman-khalid]

Two issues with read-only access in Experience Workspace:

• There's no indication in the UI that a user has read-only access. We should add a lock icon like DA has today.
• a read only user can't edit in doc-mode (correct) but in canvas, they can make phantom text updates which aren't persisted via collab to the backend (partially correct, but they should not have a false impression that they can make content edits).