Send email notification when username changed (#3022)

i18n/en.json

@@ -1203,6 +1203,18 @@
     "id": "api.templates.email_change_subject",
     "translation": "Your email address has changed for {{.TeamDisplayName}}"
   },
+  {
+    "id": "api.templates.username_change_body.info",
+    "translation": "Your username for {{.TeamDisplayName}} has been changed to {{.NewUsername}}.<br>If you did not make this change, please contact the system administrator."
+  },
+  {
+    "id": "api.templates.username_change_body.title",
+    "translation": "You updated your username"
+  },
+  {
+    "id": "api.templates.username_change_subject",
+    "translation": "Your username has changed for {{.TeamDisplayName}}"
+  },
   {
     "id": "api.templates.email_change_verify_body.button",
     "translation": "Verify Email"
@@ -1639,6 +1651,10 @@
     "id": "api.user.send_email_change_email_and_forget.error",
     "translation": "Failed to send email change notification email successfully err=%v"
   },
+  {
+    "id": "api.user.send_email_change_username_and_forget.error",
+    "translation": "Failed to send username change notification email successfully err=%v"
+  },
   {
     "id": "api.user.send_email_change_verify_email_and_forget.error",
     "translation": "Failed to send email change verification email successfully err=%v"

i18n/fr.json

@@ -1103,6 +1103,18 @@
     "id": "api.templates.email_change_subject",
     "translation": "Votre adresse électronique a été modifiée pour {{.TeamDisplayName}}"
   },
+  {
+    "id": "api.templates.username_change_body.info",
+    "translation": "Votre nom d'utilisateur pour {{.TeamDisplayName}} a été modifiée en {{.NewUsername}}.<br>Si vous n'êtes pas à l'origine de cette action, veuillez contacter votre administrateur système."
+  },
+  {
+    "id": "api.templates.username_change_body.title",
+    "translation": "Vous avez mis à jour votre nom d'utilisateur."
+  },
+  {
+    "id": "api.templates.username_change_subject",
+    "translation": "Votre nom d'utilisateur a été modifiée pour {{.TeamDisplayName}}"
+  },
   {
     "id": "api.templates.email_change_verify_body.button",
     "translation": "Vérifier votre adresse électronique"
@@ -1495,6 +1507,10 @@
     "id": "api.user.send_email_change_email_and_forget.error",
     "translation": "Impossible d'envoyer le courriel de changement d'adresse électronique err=%v"
   },
+  {
+    "id": "api.user.send_email_change_username_and_forget.error",
+    "translation": "Impossible d'envoyer le courriel de changement de nom d'utilisateur err=%v"
+  },
   {
     "id": "api.user.send_email_change_verify_email_and_forget.error",
     "translation": "Impossible d'envoyer le courriel de changement d'adresse électronique err=%v"