Messages can show up in feed when moved to muted narrow

[PIG208]

This applies to message moves that happen internally within a narrow. For example:

• In CombinedFeedNarrow, when a message (currently visible) is moved into a muted channel or muted topic;
• In ChannelNarrow, when a message (currently visible) is moved into a muted topic.

The user facing effect is that such a message will remain visible after the move, when it is not supposed to be visible in the narrow:

… until the next re-fetch from scratch.

A closely related issue, but for handling muting/unmuting channels:

• msglist: Update message list on muting/unmuting a channel #1255