Use stringex to_url to create nicer URLs from non-ASCII Marketplace name

Request:

```json
{ "marketplace_language": "de"
, "marketplace_country": "DE"
, "admin_email": "mikko+12312412123124@sharetribe.com"
, "admin_first_name": "Eddie"
, "admin_last_name": "Admin"
, "admin_password": "secret_word"
, "marketplace_type": "product"
, "marketplace_name": "Откройте для себя нечто"
}
```

Response before fix:

```json
{"marketplace_url":"http://-----------------------2.lvh.me:3000?auth=V7nnPE-z6Us”}
```

Response after fix:

```json
{"marketplace_url":"http://otkroitie-dlia-siebia-niechto3.lvh.me:3000?auth=0f5GVLop5NY"}
```