Adding a show_read override to GetPosts.

[dessalines]

• If show_read is true, it overrides the local user show_read setting.
• Fixes #4124

[dullbananas]

show_read: Some(false) doesn't override the user setting, which could cause confusion. Use Option<bool> in PostQuery.

[dessalines]

I've updated to make all of the post and comment query params as options.