Adding a show_read override to GetPosts.

[dessalines]

• If show_read is true, it overrides the local user show_read setting.
• Fixes Ignore user filters per request #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.

[dullbananas]

Changing to Option<bool> is not needed for fields other than show_read because they have the same default value every time, unlike show_read which overrides a LocalUser field.

If the inconsistency of using both bool and Option<bool> is a problem, then you can either:

• Keep these changes (except for the change to page_back in my opinion)
• Use bool instead of Option<bool>, remove the Default derive, and create this function that's does the same thing as Default::default except it sets local_user: local_user, show_read: local_user.show_read_posts():

  fn new(local_user: Option<&'a LocalUser>) -> Self