Fix panic when adding first post

[ailisp]

When a developer deploy devgigsboard to his account and try to add a post for the first time, it hits this panic:

        let mut siblings = self
            .post_to_children
            .get(&parent_id)
            .unwrap_or_else(|| panic!("Parent id {} not found", parent_id));

This PR fixes this corner case and preserve the logic for the other cases.

[frol]

@ailisp Have you considered initializing the parent post in the new function? I feel it will be more reliable that way and clear from maintenance point of view.

[ailisp]

@frol Yes that sounds better