msglist: Update message list on muting/unmuting a channel

[chrisbobbe]

The message list should update when a channel is muted or unmuted. To reproduce:

• Make sure #test here is unmuted
• Send a message in #test here on CZO, in a topic that's not muted
• Open "Combined feed" in the mobile app
• See your message there
• Mute the #test here channel (do this in the web app, pending UI for channel muting/unmuting #347)

Expected behavior: your message disappears
Actual behavior: the message does not disappear

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A resumption of part of

• Update message list on muting/unmuting a stream or topic #421

which was closed as fixed by #822, even though that PR only handled topic mutes/unmutes. It looks like we haven't revisited the channel part of #421 since then, even though there's a TODO(#421) for this in the code.

An instance of #599:

It would be nice to have an automated check for TODO(#…)s that remain in the code when they're supposed to be deleted as part of fixing the specified issue. […] [T]hey might have been missed because there was actually something important to do that wasn't done.

Related issues

• Update message list on subscribe/unsubscribe events #821

[chrisbobbe]

As part of fixing this issue, we should settle the three remaining TODO(#421)s in the code.