[WIP] Fix milestone content size

Right now creation of milestone with content size more then 255 symbols generates 500 error.
I think there is two solutions:
1) Check received content size in router and return 400 with clear error reason;
2) Change Content field from varchar(255) to text.

I suppose the second solution is better. I've changed field type, but I don't know how to create an automatic migration. 
Can anyone help me?