thunderbird: add extraConfig option

The added extraConfig option allowes users to insert custom text at
the end of the generated profile `user.js` file. This allows the users
to import templates as part of their configuration.

modules/programs/thunderbird.nix

@@ -94,12 +94,13 @@ let
       "mail.smtp.defaultserver" = "smtp_${id}";
     } // account.thunderbird.settings id;
 
-  mkUserJs = prefs: ''
+  mkUserJs = prefs: extraPrefs: ''
     // Generated by Home Manager.
 
     ${concatStrings (mapAttrsToList (name: value: ''
       user_pref("${name}", ${builtins.toJSON value});
     '') prefs)}
+    ${extraPrefs}
   '';
 in {
   meta.maintainers = with hm.maintainers; [ d-dervishi jkarlson ];
@@ -179,6 +180,14 @@ in {
                   *{scrollbar-width:none !important}
                 '';
               };
+
+              extraConfig = mkOption {
+                type = types.lines;
+                default = "";
+                description = ''
+                  Extra preferences to add to <filename>user.js</filename>.
+                '';
+              };
             };
           }));
       };
@@ -327,7 +336,8 @@ in {
           { "mail.openpgp.allow_external_gnupg" = profile.withExternalGnupg; }
 
           profile.settings
-        ] ++ (map (a: toThunderbirdAccount a profile) accounts)));
+        ] ++ (map (a: toThunderbirdAccount a profile) accounts)))
+          profile.extraConfig;
       };
     }));
   };