Move the post lookup method out of PostService

[crazytonyli]

Changes

Similar to the account lookup and the blog lookup refactor, this PR moves the function that finds a post in a blog out of PostService.

Test instructions

I don't see much risk in these changes. The lookup function implementation is almost a direct translation of the original Objective-C code, and the context objects used in call sites are not changed.

Regression Notes

1. Potential unintended areas of impact None.

2. What I did to test those areas of impact (or what existing automated tests I relied on) None.

3. What automated tests I added (or what prevented me from doing so) One simple unit test.

PR submission checklist:

• [x] I have completed the Regression Notes.
• [x] I have considered adding unit tests for my changes.
• [x] I have considered adding accessibility improvements for my changes.
• [x] I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[wpmobilebot]

You can test the changes in Jetpack from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr19317-d3c5224 on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[wpmobilebot]

You can test the changes in WordPress from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr19317-d3c5224 on your iPhone

If you need access to App Center, please ask a maintainer to add you.