trash: a delete can be undone

`x`, `dd`, `:delete` and `:rmspace` no longer end anything. What they
delete is moved, whole, under one hidden root, and `u` puts the last of it
back where it was. `:trash` lists everything deleted, the last first:
Enter or `r` restores, `x` deletes one for good and `:trash purge` empties
it, each asking first. Something whose folder has gone since comes back to
the root of its space; something whose space is in the trash too waits for
the space. The daily sweep purges what has waited WORKSPACE_TRASH_DAYS (30),
and only then are its files deleted.

While it waits, a trashed node is out of the tree, the search, the agenda,
the mails, the SFTP mount, the export and the readable files/ view, and a
shared document answers 404 until it comes back with the same link. `rm`
over the mount trashes too; a rename onto a taken name still replaces what
was there, so editor saves do not fill the trash.

.env.example

@@ -104,3 +104,7 @@ WORKSPACE_MAIL_KINDS=reminder,card_due,digest,login
 # Minutes the vault stays open with nothing using it, before it locks itself.
 # It also locks on logout and whenever the server restarts.
 WORKSPACE_VAULT_IDLE_MIN=15
+
+# Days a deleted document, folder, file or space waits in the trash, where
+# :trash can bring it back, before it and its files are deleted for good.
+WORKSPACE_TRASH_DAYS=30