Set a proper HTTP user-agent header (#9482)

Previously, mattermost-server would always request with the default
user-agent of Go's net/http package that is `Go-http-client/1.1` or
something similar.
This has several disadvantages, one is that the default user-agent
made it pretty hard to distinguish mattermost requests from other
service requests in a network log for example.

Now a user-agent of the form `mattermost-<current-version>` is set in
the client.

- [x] Added or updated unit tests (required for all new features)

services/httpservice/httpservice.go

@@ -5,7 +5,6 @@ package httpservice
 
 import (
 	"net"
-	"net/http"
 	"strings"
 
 	"github.com/mattermost/mattermost-server/services/configservice"
@@ -13,7 +12,7 @@ import (
 
 // Wraps the functionality for creating a new http.Client to encapsulate that and allow it to be mocked when testing
 type HTTPService interface {
-	MakeClient(trustURLs bool) *http.Client
+	MakeClient(trustURLs bool) *Client
 	Close()
 }
 
@@ -25,7 +24,7 @@ func MakeHTTPService(configService configservice.ConfigService) HTTPService {
 	return &HTTPServiceImpl{configService}
 }
 
-func (h *HTTPServiceImpl) MakeClient(trustURLs bool) *http.Client {
+func (h *HTTPServiceImpl) MakeClient(trustURLs bool) *Client {
 	insecure := h.configService.Config().ServiceSettings.EnableInsecureOutgoingConnections != nil && *h.configService.Config().ServiceSettings.EnableInsecureOutgoingConnections
 
 	if trustURLs {