Clean and modify interface of functions in zulip_bots/lib.py to work for Embedded bots.

[abhijeetkaur]

[zulipbot]

Heads up @abhijeetkaur, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/master branch and resolve your pull request's merge conflicts accordingly.

[timabbott]

I merged the first bit of this, after some changes to variable names etc. Want to spend some more time thinking about the rest.

[showell]

@abhijeetkaur This looks great, but you need to fix a merge conflict! Thanks!

[zulipbot]

Heads up @abhijeetkaur, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/master branch and resolve your pull request's merge conflicts accordingly.

[PIG208]

We need to merge this PR after tweaking it a little bit. As embedded bots will not work in private groups otherwise.

We also need to note that the change needs to be reflected in the server. The function is_private_message_but_not_group_pm commented that "This function is used by the embedded bot system in the zulip/zulip project, so refactor with care", but it is, in fact, not used yet. We need to do a follow-up in zerver/worker/queue_processors.py.

Yet it's stale, I think we can reuse some of the code here.