Re-organize juno specs

As discussed at our nova meetings, reorganize the juno specs into
three directories:

 - proposed: things proposed which weren't approved
 - approved: things we approved but didn't implement
 - implemented: things approved and implemented

The first I suspect is the most controversial. I've done this
because I worry about the case where a future developer wants to
pick up something dropped by a previous developer, but has trouble
finding previous proposed specifications on the topic. Note that
the actual proposed specs for Juno are adding in a later commit.

Change-Id: Idcf55ca37a83d7098dcb7c2971240c4e8fd23dc8