Use 'default' branch label instead of 'flaskbb'

Before we had to run
'flaskbb db revision -b flaskbb "my revision"'
in order to create a valid migration for flaskbb.
With this change it's possible to just run
'flaskbb db revision "my revision"'.

migrations/8ad96e49dc6_init.py

@@ -12,7 +12,7 @@ import sqlalchemy as sa
 
 revision = '8ad96e49dc6'
 down_revision = None
-branch_labels = ('flaskbb',)
+branch_labels = ('default',)
 
 
 def upgrade():