Fix memory leaks in ReaderStreamViewController

[crazytonyli]

Issue

There are many delegate type references in Reader that are not declared as weak references.

Here are steps to reproduce the memory leak issue caused by them:

1. Launch the app from Xcode
2. Go to the Reader tab -> Discover
3. Tap a topic in the "You might like" section: DIY, Baking, etc.
4. You should see a "Topic" screen at this point, which is implemented as ReaderStreamViewController.
5. Set a breakpoint at ReaderStreamViewController.deinit.
6. Exit the "Topic" screen
7. Xcode should stop at the ReaderStreamViewController.deinit breakpoint.

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 the 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)
   None.

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

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	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr21015-19c7682
	Version	22.7
	Bundle ID	org.wordpress.alpha
	Commit	19c7682
	App Center Build	WPiOS - One-Offs #6216

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📲 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	pr21015-19c7682
	Version	22.7
	Bundle ID	com.jetpack.alpha
	Commit	19c7682
	App Center Build	jetpack-installable-builds #5242

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[mokagio]

My breakpoint hit

When going from the default Read home screen, "Following" to "Discover":

When going from back from "Discover" > Tap "Nature" > "Nature" ReaderStreamViewController to "Discover":

[mokagio]

Taking over merging to make sure this will land on trunk before the upcoming 22.8 code freeze