Bug 1100860 - [email/UI] message_reader should filter move targets

The message_list card already does this.  Without this fix it's possible
to break the back-end, at least for ActiveSync.  Spinning off a separate
bug to harden the back-end, specifically ActiveSync.
Merge pull request #26250 from asutherland/email-reader-move-pred

Bug 1100860 - [email/UI] message_reader should filter move targets. r=jrburke
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