Properly disable autoincrement in assoc table

- for the chatroom_id FK it's already done automatically by SQLAlchemy
- for event_id (not a FK yet) it should be specified in the model, too

chat/indico_chat/migrations/201409291524_1bd6c5129d29_create_tables.py

@@ -35,8 +35,7 @@ def upgrade():
     op.create_table('chatroom_events',
                     sa.Column('event_id', sa.Integer(), nullable=False, primary_key=True, index=True,
                               autoincrement=False),
-                    sa.Column('chatroom_id', sa.Integer(), nullable=False, primary_key=True, index=True,
-                              autoincrement=False),
+                    sa.Column('chatroom_id', sa.Integer(), nullable=False, primary_key=True, index=True),
                     sa.Column('hidden', sa.Boolean(), nullable=False),
                     sa.Column('show_password', sa.Boolean(), nullable=False),
                     sa.ForeignKeyConstraint(['chatroom_id'], ['plugin_chat.chatrooms.id']),

chat/indico_chat/models/chatrooms.py

@@ -114,7 +114,8 @@ class ChatroomEventAssociation(db.Model):
     event_id = db.Column(
         db.Integer,
         primary_key=True,
-        index=True
+        index=True,
+        autoincrement=False
     )
     #: ID of the chatroom
     chatroom_id = db.Column(