refactor:Extract function for message being in a narrow

[neiljp]

This could do with a unit test, though it's just a refactor right now and the tests would potentially cover a large state space.

I'm thinking this could be useful in many ways right now already as an available method that could be used in various places.

@zee-bit Could this be useful for #824 if you constructed a fake message dict to pass in?

[zulipbot]

Hello @zulip/server-refactoring members, this pull request was labeled with the "area: refactoring" label, so you may want to check it out!

[zee-bit]

@neiljp Thanks for opening this PR. This could potentially be useful for #824, considering I was currently constructing a fake narrow list there, instead of message dict. I can definitely see how we can construct a message dict(from request and narrow) as an argument for this function as well. For simplicity and reusability, I can make a function that constructs a fake message dict using requests & narrow, and then pass its output to this function.

However, I still see both the functions to be particularly similar, considering the only difference would be that the function in #824 has its use-case limited to just notifying the user if the message is not from the same narrow, whereas this function could be used in various places that need this particular logic including that PR. Is there any other difference that I may have missed?