Migrated get_next_unread_pm and tests to model.

[WladRamos]

What does this PR do, and why?

This pull request Migrates get_next_unread_pm & tests to model. Created this new version to the pull request to make it cleaner by splitting the changes into two commits. This is a precursor for improving the get_next_unread_pm method. Fixes #1335

External discussion & connections

• [ ] Discussed in #zulip-terminal in topic
• [x] Fully fixes #1335
• [ ] Partially fixes issue #
• [ ] Builds upon previous unmerged work in PR #
• [ ] Is a follow-up to work in PR #
• [ ] Requires merge of PR #
• [x] Merge will enable work on #1336

How did you test this?

• [ ] Manually - Behavioral changes
• [ ] Manually - Visual changes
• [ ] Adapting existing automated tests
• [x] Adding automated tests for new behavior (or missing tests)
• [x] Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• [x] It is a minimal coherent idea
• [x] It has a commit summary following the documented style (title & body)
• [x] It has a commit summary describing the motivation and reasoning for the change
• [x] It individually passes linting and tests
• [ ] It contains test additions for any new behavior
• [ ] It flows clearly from a previous branch commit, and/or prepares for the next commit

[zulipbot]

Hello @WladRamos, it seems like you have referenced #1335 in your pull request description, but you have not referenced them in your commit message description(s). Referencing an issue in a commit message automatically closes the corresponding issue when the commit is merged, which makes the issue tracker easier to manage.

Please run git commit --amend in your command line client to amend your commit message description with Fixes #1335..

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.

To learn how to write a great commit message, please refer to our guide.

[neiljp]

@WladRamos I just merged a slightly trimmed version of your first commit, and the other parts back to this PR. I'm going to make some notes in a moment.

[zulipbot]

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

[WladRamos]

@neiljp Thanks for the feedback, I'll take a look. 👍

[neiljp]

As noted in #1335, the migration was complete in the merged commit, but this remains a useful refactor as part of #1336, so marking this as a PR completion candidate.