State: Reset editor undo history only at initial setup

[aduth]

Fixes #2916
Fixes #2915

This pull request seeks to revise the action types on which the editor undo history is reset. The current RESET_POST reset type is insufficient for two reasons:

• It wipes history when an autosave occurs (Auto save wipes undo levels #2915)
• Other changes occur during initial setup that flag history on a new post (Undo level present on load #2916)

By changing the reset type to SETUP_EDITOR, we resolve both of these issues by only resetting at the final setup action during editor initialization.

Testing instructions:

Repeat testing instructions from #2916 and #2915, verifying that you can:

• Undo after a save occurs
• No undo when starting a new post