Ensure Turbo mode works seamlessly with force_uncompressed_tinymce in WP 5.0

[westonruter]

See phpDoc for force_uncompressed_tinymce in https://core.trac.wordpress.org/changeset/43738:

* Force uncompressed TinyMCE when a custom theme has been defined.
*
* The compressed TinyMCE file cannot deal with custom themes, so this makes
* sure that we use the uncompressed TinyMCE file if a theme is defined.
* Even if we are on a production environment.

With Turbo mode (#42) in the PWA plugin (where admin assets are precached) then this should be irrelevant. In other words, custom themes should not have a negative performance impact. This should be confirmed.