Previously topics weren't being highlighted at all and messages had their
highlighting persist across different narrows (because we were only checking
whether the message object had a match_content property, not whether it should
currently be used).
(imported from commit 44c91c6d5799dcdf765e19e1a17bd727ce80c918)
Now that we no longer use tables for our message list, we can
more logically group messages together.
(imported from commit 9923a092f91a45fe3ef06f2f00e23e4e3fb62a37)