IVPN server data update code and ISP filter (#578)

- Use IVPN's HTTP API instead of their .zip file
- Unit tests for API and GetServers
- Paves the way for Wireguard
- Update server information for IVPN
- Add `ISP` filter for IVPN

internal/models/server.go

@@ -41,6 +41,7 @@ type IpvanishServer struct {
 type IvpnServer struct {
 	Country  string   `json:"country"`
 	City     string   `json:"city"`
+	ISP      string   `json:"isp"`
 	Hostname string   `json:"hostname"`
 	TCP      bool     `json:"tcp"`
 	UDP      bool     `json:"udp"`