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)

utils/testutils/mocked_http_service.go

@@ -4,6 +4,7 @@
 package testutils
 
 import (
+	"github.com/mattermost/mattermost-server/services/httpservice"
 	"net/http"
 	"net/http/httptest"
 )
@@ -18,8 +19,8 @@ func MakeMockedHTTPService(handler http.Handler) *MockedHTTPService {
 	}
 }
 
-func (h *MockedHTTPService) MakeClient(trustURLs bool) *http.Client {
-	return h.Server.Client()
+func (h *MockedHTTPService) MakeClient(trustURLs bool) *httpservice.Client {
+	return &httpservice.Client{Client: h.Server.Client()}
 }
 
 func (h *MockedHTTPService) Close() {