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)

app/post.go

@@ -16,12 +16,14 @@ import (
 	"strings"
 
 	"github.com/dyatlov/go-opengraph/opengraph"
+	"golang.org/x/net/html/charset"
+
 	"github.com/mattermost/mattermost-server/mlog"
 	"github.com/mattermost/mattermost-server/model"
 	"github.com/mattermost/mattermost-server/plugin"
+	"github.com/mattermost/mattermost-server/services/httpservice"
 	"github.com/mattermost/mattermost-server/store"
 	"github.com/mattermost/mattermost-server/utils"
-	"golang.org/x/net/html/charset"
 )
 
 func (a *App) CreatePostAsUser(post *model.Post, clearPushNotifications bool) (*model.Post, *model.AppError) {
@@ -919,7 +921,7 @@ func (a *App) DoPostAction(postId, actionId, userId, selectedOption string) *mod
 	req.Header.Set("Accept", "application/json")
 
 	// Allow access to plugin routes for action buttons
-	var httpClient *http.Client
+	var httpClient *httpservice.Client
 	url, _ := url.Parse(action.Integration.URL)
 	siteURL, _ := url.Parse(*a.Config().ServiceSettings.SiteURL)
 	subpath, _ := utils.GetSubpathFromConfig(a.Config())