manager: The big refactoring

Clean up the logic in main to make it more readable.  Factor out common
functionality into the respective modules.

techtree-manager/src/tree.rs

@@ -166,11 +166,14 @@ impl Tree {
             &|_g, (_, element)| element.to_dot_node_attributes(None),
         );
 
-        format!(r#"digraph {{
+        format!(
+            r#"digraph {{
     ranksep=1.2
 {:?}
 }}
-"#, dot)
+"#,
+            dot
+        )
     }
 
     pub fn to_mermaid(&self, repo_url: &str) -> String {