Explicitly declare length of large MySQL indices.

Most MySQL installs silently truncate indices at a specific number of bytes.
This value is 767 bytes unless either the innodb_large_prefix option is turned
on or the innodb_page_size has been changed. This change explicitly limits the
size of large indices to 767 characters due to the latin1 charset being 1 byte
per char.

data/migrations/versions/228d1af6af1c_mysql_max_index_lengths.py

@@ -0,0 +1,22 @@
+"""mysql max index lengths
+
+Revision ID: 228d1af6af1c
+Revises: 5b84373e5db
+Create Date: 2015-01-06 14:35:24.651424
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '228d1af6af1c'
+down_revision = '5b84373e5db'
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import mysql
+
+def upgrade(tables):
+  op.drop_index('queueitem_queue_name', table_name='queueitem')
+  op.create_index('queueitem_queue_name', 'queueitem', ['queue_name'], unique=False, mysql_length=767)
+
+def downgrade(tables):
+  pass