Message adjustments for editing posts.

Previously, when you edited a post and clicked "Preview", you would
have two "Edit" buttons next to one another, one which meant
"keep editing" and one which meant "Save changes".

I've changed the keep editing button to actually say "Keep editing"
and the save button to actually say "Save changes".

Change-Id: I5c51dc45430e75b03900a61c71d5419391dd77c0
Merge "Message adjustments for editing posts." into frontend-rewrite