Clean up the "anchor" values around the message list #3883
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This isn't really contributing anything useful to add to the layers immediately above and below it. The name is pretty mysterious about what it means, too. We'll clean up some other names around here, but easiest to have one fewer to deal with.
This is rather more explicit about what this selector actually means. To the extent that "anchor" conveys any meaning of its own, it's an answer to a UI question of how the message list should behave. And that's a question whose answer should live in the MessageList UI code, not in a selectors module.
This is an example of zulip#3015.
Zero is never a great choice for a "no such item" sentinel where a `number` is expected -- it makes it likely the sentinel will be implicitly treated like just another number. Here, it turns out that where the value ultimately gets consumed (in `scrollToAnchor`), it has the effect of causing no message node to be found... so that we scroll to the *bottom*. Make that path through the control flow explicit. In this case, 0 is an especially confusing value to use for "there is no unread message" because this value is called "anchor"... and in other places called "anchor", we use 0 (as FIRST_UNREAD_ANCHOR) to mean the first unread message!
This makes it a lot more explicit what this means -- what an "anchor" has to do with the display of the message list.
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This clears up a swath of the confusion around the concept of "anchor" that I dug into following #3877 .