PLT-3647 Email Batching (#3718)

* PLT-3647 Added config settings for email batching

* PLT-3647 Refactored generation of email notification

* PLT-3647 Added serverside code for email batching

* PLT-3647 Updated settings UI to enable email batching

* PLT-3647 Removed debug code

* PLT-3647 Fixed 0-padding of minutes in batched notification

* PLT-3647 Updated clientside UI for when email batching is disabled

* Go fmt

* PLT-3647 Changed email batching to be disabled by default

* Updated batched email message

* Added email batching toggle to system console

* Changed Email Notifications > Immediate setting to a 30 second batch interval

* Go fmt

* Fixed link to Mattermost icon in batched email notification

* Updated users to use 30 second email batching by default

* Fully disabled email batching when clustering is enabled

* Fixed email batching setting in the system console

* Fixed casing of 'Send Email notifications' -> 'Send email notifications'

* Updating UI Improvements for email batching (#3736)

* Updated text for notification settings and SiteURL.

* Prevented enabling email batching when SiteURL isn't set in the system console

* Re-added a couple debug messages

* Added warning text when clustering is enabled

api/post_test.go

@@ -1147,3 +1147,42 @@ func TestGetFlaggedPosts(t *testing.T) {
 		t.Fatal("should not have gotten a flagged post")
 	}
 }
+
+func TestGetMessageForNotification(t *testing.T) {
+	Setup()
+	translateFunc := utils.GetUserTranslations("en")
+
+	post := &model.Post{
+		Message:   "test",
+		Filenames: model.StringArray{},
+	}
+
+	if getMessageForNotification(post, translateFunc) != "test" {
+		t.Fatal("should've returned message text")
+	}
+
+	post.Filenames = model.StringArray{"test1.png"}
+	if getMessageForNotification(post, translateFunc) != "test" {
+		t.Fatal("should've returned message text, even with attachments")
+	}
+
+	post.Message = ""
+	if message := getMessageForNotification(post, translateFunc); message != "1 image sent: test1.png" {
+		t.Fatal("should've returned number of images:", message)
+	}
+
+	post.Filenames = model.StringArray{"test1.png", "test2.jpg"}
+	if message := getMessageForNotification(post, translateFunc); message != "2 images sent: test1.png, test2.jpg" {
+		t.Fatal("should've returned number of images:", message)
+	}
+
+	post.Filenames = model.StringArray{"test1.go"}
+	if message := getMessageForNotification(post, translateFunc); message != "1 file sent: test1.go" {
+		t.Fatal("should've returned number of files:", message)
+	}
+
+	post.Filenames = model.StringArray{"test1.go", "test2.jpg"}
+	if message := getMessageForNotification(post, translateFunc); message != "2 files sent: test1.go, test2.jpg" {
+		t.Fatal("should've returned number of mixed files:", message)
+	}
+}