Rename "All messages" to "Combined feed" in our codebase

[gnprice]

This is a followup to:

• Rename "All messages" to "Combined feed" #634

That issue was for updating the UI. We should also update all the references to this Zulip feature within our code.

A good search to use for this is:
git grep -iP 'all.?messages'
That will find the phrase "all messages" with any capitalization, and with or without a space (or other character) in between the words. Currently it finds 73 results.

When this issue is complete, that search's results won't necessarily be empty — there may be a few matches that aren't referring to the former "All messages" narrow, now called "Combined feed". But it should be possible to look at each of the remaining results and determine how that's not what it's referring to.

For this issue, the most important part of the work is making a highly reviewable PR. Be sure to review that page of the Zulip docs, and the page on clear and coherent commits.