[Backport to 12.6] Disable "Go to content perspective" button while the visual editor is loading

On slow connections/servers, it is possible to open the content-perspective editor from the visual editor, while the visual editor is still loading. When this happens, we don't cleanup the editable instance, and we end up in edit-mode, which is not intended. The "Go to content perspective" button should only be enabled when the visual editor has finished loading, or if an error occurs that does not explicitly signal the button to be disabled (this is existing behaviour).

Scenario:

1.  Open any page in Channel Manager.
2.  Using Chrome network throttling dev tool, set internet connection to  3g.
3.  Open a document in the visual editor and before fields  are loaded click "Go to content perspective button."

Problem: The document is opened in edit mode.

Expected result: Document should be open in View mode.