[Upstream] Fix for tinymce editor not being initialized on richtext fields

[astratagem]

Cherry-picked version of alleyinteractive#628 without all the new stuff so we can avoid deprecation warnings for Fieldmanager_Context_Page (see alleyinteractive#157).

QA

You'll have to check this out locally:

# From your local environment root
cd wp-content/plugins/wordpress-fieldmanager
git fetch
git checkout spiritedmedia-collapsed-richtext-tinymce-reinit

Is the issue described in https://github.com/spiritedmedia/spiritedmedia/pull/2591#issuecomment-388175593 fixed?

[danielbachhuber]

👋

(I somehow was subscribed to this repo, so thought I'd say hello)

[astratagem]

Hello! Thanks for dropping by!

[kingkool68]

👍Works as intended.