fixed initial setup for mod mail category

fixed setup for locking
added exception message in case the modmail post target is not defined

abstracto-application/abstracto-modules/modmail/modmail-impl/src/main/resources/templates/exception/modmail_exception_post_target_not_defined_embed_en_US.ftl

@@ -0,0 +1,12 @@
+{
+  "author": {
+    "name": "${user.member.effectiveName}",
+    "avatar":  "${user.member.user.effectiveAvatarUrl}"
+  },
+  "color" : {
+    "r": 200,
+    "g": 0,
+    "b": 255
+  },
+  "description": "<#include "modmail_post_target_error">"
+}