UTF-8 v1_json_metadata, comment, manifest

This will allow us to store unicode JSON blobs in the column on MySQL.

util/migrate/__init__.py

@@ -0,0 +1,16 @@
+from sqlalchemy.types import TypeDecorator, Text
+from sqlalchemy.dialects.mysql import TEXT as MySQLText, LONGTEXT
+
+class UTF8LongText(TypeDecorator):
+  """ Platform-independent UTF-8 LONGTEXT type.
+
+  Uses MySQL's LongText with charset utf8mb4, otherwise uses TEXT, because
+  other engines default to UTF-8 and have longer TEXT fields.
+  """
+  impl = Text
+
+  def load_dialect_impl(self, dialect):
+    if dialect.name == 'mysql':
+      return dialect.type_descriptor(LONGTEXT(charset='utf8mb4', collation='utf8mb4_unicode_ci'))
+    else:
+      return dialect.type_descriptor(Text())