BUG Flush editors on form submission (os#8222)

Otherwise some non-textual changes such as moving images and resizing
them could be missed when saving.

This is because TinyMCE does not treat the image move/resize as an
undolevel and does not register this via regular "onChange" event.
Merge pull request #1144 from silverstripe-droptables/os8222

BUG Flush editors on form submission (os#8222)