Fixed help topics not working right when multiple factions share an id

For example the factions of Default Era and Age of Heroes have the same id's, which caused the faction help topics to have the same id's as well, leading to a faction link on both era pages to lead to the same faction page. Adding an era prefix to the faction topic id's makes them unique and solves the problem.

src/help/help_impl.cpp

@@ -656,7 +656,7 @@ std::vector<topic> generate_faction_topics(const config & era, const bool sort_g
 		}
 
 		const std::string name = f["name"];
-		const std::string ref_id = faction_prefix + id;
+		const std::string ref_id = era_prefix + era["id"] + "_" + faction_prefix + id;
 		topics.emplace_back(name, ref_id, text.str());
 	}
 	if (sort_generated)