(bug 43312) Add messages for explanatory text to editpage form

Adds messages templatesandbox-editpage-text to display above the form,
and templatesandbox-editpage-helptext to display after the preview
button.

Also updates message use to use Message::isDisabled() rather than
testing if the message text winds up empty.

Change-Id: I9d5698c7a05a999ddf59d1f841a83d486383a3ea
Merge "(bug 43312) Add messages for explanatory text to editpage form"