Increase DataSet name length from 50 to 200

We had a dataset in Backdrop which was 53 characters, so the import script
failed. This only because obvious when we started using PostgreSQL in
development.

Note that this will be our first schema migration that (should) happen 
automatically on deploy, so we ought to check that the preview database 
changes after we merge :)

See https://www.pivotaltracker.com/story/show/67218992
[Fixes #67218992]