Add customizable feed card divider

[hjiangsu]

Pull Request Description

This PR adds a new way to customize the divider between posts/comments in the feed page. You can customize both the height and the colour of the divider. As part of this, I also made some tweaks to make some parameters optional in ListPickerItem and ListOption

Issue Being Fixed

This is related to some feedback from a post in the Thunder community: https://lemmy.world/post/13333300

Issue Number: N/A

Screenshots / Recordings

95441758-1095-4baf-9748-104dd1eabce9.mp4

Checklist

• Did you update CHANGELOG.md?
• Did you use localized strings where applicable?
• Did you add semanticLabels where applicable for accessibility?

[micahmo]

Very nice! I like the new options for settings. We could probably use a similar technique to clean up and group other settings.