Add validation for x10 fragment size

This changes:

- Updates seeds to use HiSequ X, not HiSeq Xten to be consistent with migrations and production system.
- Made matching change to the migration itself.
- Merged in production (hence the deletion of the triggers migration)
- Fragment sizes are stored as strings, not integers. Not sure why this is the case, but I've updated the validators to correspond.
- Commited the updated schema.rb

The interface still presents a free text field, rather than pre-populating it and locking it. Its probably not worth fixing this for this story, as Xten stuff will be going through bulk submission anyway.