Add validation to salary field on job submission board

[DavidHernandez]

The salary field is required on the job submission board. A lot of companies, do not want to publish the salary, so they add a text saying like "salary depends on the person". To prevent this, we should have some kind of validation, like checking that it has numbers on it. But we can not forbid the text on the input, as some people would write things like "between 30 to 40k".