Add support for a fixed_irc_server configuration

This option lets the administrator choose a specific IRC server, and only
that server can be used with this biboumi instance.

In this mode, JIDs to use are changed like this:

- #chan%irc.example.com@biboumi.example.com      -> #chan@biboumi.example.com
- user!irc.example.com@biboumi.example.com       -> user!@biboumi.example.com
- #chan%irc.example.com@biboumi.example.com/Nick -> #chan@biboumi.example.com/Nick
- %irc.example.com@biboumi.example.com           -> no equivalent
- irc.example.com@biboumi.example.com            -> no equivalent

src/utils/empty_if_fixed_server.hpp

@@ -0,0 +1,26 @@
+#ifndef EMPTY_IF_FIXED_SERVER_HPP_INCLUDED
+#define EMPTY_IF_FIXED_SERVER_HPP_INCLUDED
+
+#include <string>
+
+#include <config/config.hpp>
+
+namespace utils
+{
+  inline std::string empty_if_fixed_server(std::string&& str)
+  {
+    if (!Config::get("fixed_irc_server", "").empty())
+      return {};
+    return str;
+  }
+
+  inline std::string empty_if_fixed_server(const std::string& str)
+  {
+    if (!Config::get("fixed_irc_server", "").empty())
+      return {};
+    return str;
+  }
+
+}
+
+#endif /* EMPTY_IF_FIXED_SERVER_HPP_INCLUDED */