Handle internal PM links

[Ezio-Sarthak]

This is a follow-up PR from #708. It adds support for handling internal PM links.

I highly appreciate the work of @preetmishra in #708, by which it became simpler and cleaner to work with the helper functions in general :)

Partially fixes #764.

[zulipbot]

Hello @Ezio-Sarthak, it seems like you have referenced #764 in your pull request description, but you have not referenced them in your commit message description(s). When you reference an issue in a commit message, it automatically closes the corresponding issue when the commit is merged.

Please run git commit --amend in your command line client to amend your commit message description with Fixes #764..

An example of a correctly-formatted commit:

commit fabd5e450374c8dde65ec35f02140383940fe146
Author: zulipbot
Date:   Sat Mar 18 13:42:40 2017 -0700

    pull requests: Check PR commits reference when issue is referenced.

    Fixes #51.

Thank you for your contributions to Zulip!

[neiljp]

@Ezio-Sarthak This seems like it would be a fairly straightforward PR to get in, though:

• this needs rebasing over the black transition?
• this doesn't always narrow to the right message - this may be due to this PR being at a state prior to a fix that we merged later?

[zulipbot]

Heads up @Ezio-Sarthak, we just merged some commits that conflict with the changes you 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/main branch and resolve your pull request's merge conflicts accordingly.