Switch JSONEncoded from VARCHAR to TEXT

The JSON fields are really "dumb stuff in here" fields.
They are not intended to get indexed or anything. And they
can get large. For example the exif_all field in one of my
simple tests is nearly 7 kB large. Although VARCHAR might
work, TEXT feels just better as the storage type.

mediagoblin/db/sql/extratypes.py

@@ -15,7 +15,7 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 
-from sqlalchemy.types import TypeDecorator, Unicode, VARCHAR
+from sqlalchemy.types import TypeDecorator, Unicode, TEXT
 import json
 
 
@@ -50,7 +50,7 @@ class PathTupleWithSlashes(TypeDecorator):
 class JSONEncoded(TypeDecorator):
     "Represents an immutable structure as a json-encoded string."
 
-    impl = VARCHAR
+    impl = TEXT
 
     def process_bind_param(self, value, dialect):
         if value is not None: