Be clearer about why new amendments can't be created

One one amendment can be created per post. If one is already created, the "create amendment" link just disappears, which can be confusing if the post is trashed, since it then also won't appear in the list.

One potential solution is to show "Create Amendment" as a disabled link with a tooltip informing the user. Additionally (or alternatively), showing a "Edit draft amendment" link to the already extant amendment would be useful.