Change contest config for required contest from comma separated string to YAML list of strings

src/contestreader_yaml.rs

@@ -27,7 +27,7 @@ struct ContestYaml {
     public_listing: Option<bool>,
 
     requires_login: Option<bool>,
-    requires_contest: Option<String>,
+    requires_contest: Option<Vec<String>>,
     secret: Option<String>,
     message: Option<String>,
 
@@ -63,7 +63,7 @@ pub fn parse_yaml(content: &str, filename: &str, directory: &str) -> Option<Cont
                      config.max_grade,
                      config.position,
                      config.requires_login,
-                     config.requires_contest,
+                     config.requires_contest.map(|list| list.join(",")),
                      config.secret,
                      config.message);
     // TODO: Timeparsing should fail more pleasantly (-> Panic, thus shows message)