Highlight last tapped post

[micahmo]

Pull Request Description

This PR sort of goes along with #1520. It's a separate feature, but it's something that I realized is even more important once we have the ability to navigate out and back into posts. It introduces a small highlight color to the last selected post. This is super handy when you navigate out to check the title or media, for example, and then you want to go back into the comments. This lets you see what post you were previously looking at very easily. It's also handy when you accidentally go into a post and want to mark it as unread.

Notes

• All of the other contents respect the read status, so it is still easy to tell whether a post is read or unread, even when it has this highlight.
• The highlighting goes away when refreshing the feed.

This is another feature inspired by Relay, and I tried to match the implementaiton.

Issue Being Fixed

Issue Number: N/A

Screenshots / Recordings

qemu-system-x86_64_sAAmZF9UIa.mp4

Checklist

• If a new package was added, did you ensure it uses an appropriate license and is actively maintained?
• Did you use localized strings (and added appropriate descriptions) where applicable?
• Did you add semanticLabels where applicable for accessibility?

[hjiangsu]

LGTM, I just rebased this to be up to date with the latest develop branch!

[micahmo]

Thanks for merging this up to date!