Be consistent in aligning channel icon with channel name

PR #1877 will add a function channelTopicLabelSpan as a canonical way to produce a span of text with a channel icon plus name, and optionally a topic. E.g.:

That PR only uses it in the channel and topic action sheet headers. But there are other places that make good candidates for using it:

• Channel rows in the Inbox, Channels, and All channels pages
• The channel name in a message-list app bar
• Recipient headers in the message list

Related issues:

• #1907
• #1908