Fix memory leaks on viewing a post from a notification

[crazytonyli]

Relates to #20989.

Issue

When done viewing a post from the Notifications tab, the post detail view isn't released.

To reproduce this issue:

1. Put breakpoints at viewDidLoad and deinit functions in ReaderDetailViewController.
2. Launch the app from Xcode.
3. Go to the Notifications tab.
4. Tap a notification that says " posted on ", or any other kinds of notification that takes you to ReaderDetailViewController.
5. Xcode should hit the breakpoint at ReaderDetailViewController.viewDidLoad. This step is just to confirm that the ReaderDetailViewController has been created
6. Exit the post details screen.
7. Xcode should hit the breakpoint at ReaderDetailsViewController.deinit.

Performing the above steps on the trunk branch doesn't hit the deinit breakpoint, which means the view controller is not released.

You can test this PR by repeating the same steps on this PR branch and verify that Xcode stops at deinit breakpoint.

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)
   Viewing notifications and posts.

3. What automated tests I added (or what prevented me from doing so)
   None.

PR submission checklist:

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

UI Changes testing checklist: N/A

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr21013-e44432e
	Version	22.7
	Bundle ID	com.jetpack.alpha
	Commit	e44432e
	App Center Build	jetpack-installable-builds #5238

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr21013-e44432e
	Version	22.7
	Bundle ID	org.wordpress.alpha
	Commit	e44432e
	App Center Build	WPiOS - One-Offs #6212

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[mokagio]

Tested on Simulator.

viewDidLoad called ✅

deinit called

[mokagio]

Did you chose unowned so the type didn't have to become wrapped in Optional?

[crazytonyli]

Indeed. Given this is a "presenter" type, it should not use the view controller reference if the view controller is released. If this unowned reference causes a runtime crash in the production app, I think we may have other issues.