Fix Gmail spam/trash syncback actions.

Summary:
In the vaguely Rube-Goldberg-esque world that is Gmail IMAP, either of the
following two actions will actually move a UID to the trash folder:
* copy the UID to the '[Gmail]/Trash' [1] folder
* add the '\\Trash' label.

However, we were adding a '[Gmail]/Trash' *label*, which does *not* actually
remove the UID from the All Mail folder or move it to the trash folder.

Finally, removing a thread from trash or spam wouldn't work either, because you
have to select the appropriate folder first.

(Similar observations apply to spam.)

Mostly fixes T628, in a roundabout way.

\[1\] Or '[Gmail]/Papierkorb' or whatever, depending on your localization.

Test Plan: Check that syncback actions actually do what they promise.

Reviewers: spang, kav-ya

Reviewed By: kav-ya

Maniphest Tasks: T628

Differential Revision: https://review.inboxapp.com/D912