Update email notification subject line and contents for Group Messages (#8689)

Reordered notification strings

MM-10335 Changed uppercase CHANNEL to Lowercase Channel, added @ sign before username on notifications

Added @ sign in front of username in all email notifications.
Capitalized Direct Message and Group Message in email notifications.
Fixed the issue with long group message names.

Removed executable bit

i18n/en.json

@@ -3776,11 +3776,11 @@
   },
   {
     "id": "app.notification.body.intro.direct.full",
-    "translation": "You have a new direct message."
+    "translation": "You have a new Direct Message."
   },
   {
     "id": "app.notification.body.intro.direct.generic",
-    "translation": "You have a new direct message from {{.SenderName}}"
+    "translation": "You have a new Direct Message from @{{.SenderName}}"
   },
   {
     "id": "app.notification.body.intro.notification.full",
@@ -3788,11 +3788,19 @@
   },
   {
     "id": "app.notification.body.intro.notification.generic",
-    "translation": "You have a new notification from {{.SenderName}}"
+    "translation": "You have a new notification from @{{.SenderName}}"
+  },
+  {
+    "id": "app.notification.body.intro.group_message.full",
+    "translation": "You have a new Group Message."
+  },
+  {
+    "id": "app.notification.body.intro.group_message.generic",
+    "translation": "You have a new Group Message from @{{.SenderName}}"
   },
   {
     "id": "app.notification.body.text.direct.full",
-    "translation": "{{.SenderName}} - {{.Hour}}:{{.Minute}} {{.TimeZone}}, {{.Month}} {{.Day}}"
+    "translation": "@{{.SenderName}} - {{.Hour}}:{{.Minute}} {{.TimeZone}}, {{.Month}} {{.Day}}"
   },
   {
     "id": "app.notification.body.text.direct.generic",
@@ -3800,20 +3808,36 @@
   },
   {
     "id": "app.notification.body.text.notification.full",
-    "translation": "CHANNEL: {{.ChannelName}}<br>{{.SenderName}} - {{.Hour}}:{{.Minute}} {{.TimeZone}}, {{.Month}} {{.Day}}"
+    "translation": "Channel: {{.ChannelName}}<br>@{{.SenderName}} - {{.Hour}}:{{.Minute}} {{.TimeZone}}, {{.Month}} {{.Day}}"
   },
   {
     "id": "app.notification.body.text.notification.generic",
     "translation": "{{.Hour}}:{{.Minute}} {{.TimeZone}}, {{.Month}} {{.Day}}"
   },
+  {
+    "id": "app.notification.body.text.group_message.full",
+    "translation": "Channel: {{.ChannelName}}<br>@{{.SenderName}} - {{.Hour}}:{{.Minute}} {{.TimeZone}}, {{.Month}} {{.Day}}"
+  },
+  {
+    "id": "app.notification.body.text.group_message.generic",
+    "translation": "{{.Hour}}:{{.Minute}} {{.TimeZone}}, {{.Month}} {{.Day}}"
+  },
   {
     "id": "app.notification.subject.direct.full",
-    "translation": "[{{.SiteName}}] New Direct Message from {{.SenderDisplayName}} on {{.Month}} {{.Day}}, {{.Year}}"
+    "translation": "[{{.SiteName}}] New Direct Message from @{{.SenderDisplayName}} on {{.Month}} {{.Day}}, {{.Year}}"
   },
   {
     "id": "app.notification.subject.notification.full",
     "translation": "[{{ .SiteName }}] Notification in {{ .TeamName}} on {{.Month}} {{.Day}}, {{.Year}}"
   },
+  {
+    "id": "app.notification.subject.group_message.full",
+    "translation": "[{{ .SiteName }}] New Group Message in {{ .ChannelName}} on {{.Month}} {{.Day}}, {{.Year}}"
+  },
+  {
+    "id": "app.notification.subject.group_message.generic",
+    "translation": "[{{ .SiteName }}] New Group Message on {{.Month}} {{.Day}}, {{.Year}}"
+  },
   {
     "id": "app.plugin.activate.app_error",
     "translation": "Unable to activate extracted plugin."