Move tinymce out of non-activeadmin contexts

[camillevilla]

JIRA issue link

N/A

Description - what does this code do?

Moves tinymce out of our application.js file so that it stops trying to initialize on irrelevant pages.
https://github.com/spohlenz/tinymce-rails

Testing done - how did you test it/steps on how can another person can test it

1. Go to /admin/practices and send an email to innovation editors. Confirm that tinymce still initializes for message previews.
2. Create a PageBuilder page and confirm that components (e.g. Body text) still initialize the tinymce WYSIWYG editor.