Improve Username Constraints / Validations

[cycomachead]

There used to be validations on dom_username, but they were removed:

https://github.com/bromagosa/snapCloud/commit/6990be17040d9407addeaf650e0f3dbe01c004be#diff-bef302d3cdd1ce8c0d4e2e008fa3652a

If there's no validations, then should we not just use the text type? (It would make writing the initial migrations in #10 Easier).