Added search and filtering

As I have filled the database with 150-200 offers (number varies week-to-week as some offers are expiring and I'm adding new ones) I found it more and more difficult to navigate between different types of offers - be it employment type or seniority level.

It took me some time to figure out what would be the best way to do so, as I've seen many job sites with too complicated filtering options and I've decided to go simple in the beginning (I will adjust it based on feedback of course)

Oh, and by the way - today Work in Green got featured on 10words.io