mv embeddable types into model/embeddable.rb. Humanize names.

Change wording on XHTML embeddable to be clearer to non-technical users.

[#55991208]
https://www.pivotaltracker.com/story/show/55991208
rename text area ➙ text box.
Merge branch 'master' into staging

* master:
  rename text area ➙ text box.
  mv embeddable types into model/embeddable.rb. Humanize names.