forbid mentions in sigs

app/__init__.py

@@ -8,6 +8,7 @@ from .constants import (
     PermissionLevel, permission_level_string,
     InfoboxKind, InfoboxHTMLClass,
     REACTION_EMOJI, MOTD_BANNED_TAGS,
+    SIG_BANNED_TAGS,
     )
 from .lib.babycode import babycode_to_html, EMOJI, BABYCODE_VERSION
 from datetime import datetime
@@ -177,6 +178,7 @@ def create_app():
             "__emoji": EMOJI,
             "REACTION_EMOJI": REACTION_EMOJI,
             "MOTD_BANNED_TAGS": MOTD_BANNED_TAGS,
+            "SIG_BANNED_TAGS": SIG_BANNED_TAGS,
         }
 
     @app.context_processor