Save draft cover messages

DB migration to rename pending -> draft.  Keep pending column on
messages to mean "ready to upload".  Indicate whether messages are
drafts in change display.  Update display of (draft) messages when
their contents change.  Also store draft votes, but don't display
them until finalized (pending upload).  Also remove 'drafts' flag
from revisions if that revision has a pending upload.

Change-Id: Iff427c0c1664351ce8e2d61be2f79f3bfa1f989d