feat: Translate frontend/src/i18n/en.json in bg

91% of minimum 50% translated source file: 'frontend/src/i18n/en.json'
on 'bg'.

Sync of partially translated files: 
untranslated content is included with an empty translation 
or source language content depending on file format

frontend/src/i18n/bg.json

@@ -241,7 +241,7 @@
     "userCreated": "Потребителя е създаден!",
     "userDefaults": "Настройки по подразбиране на потребителя",
     "userDeleted": "Потребителя е изтрит!",
-    "userManagement": "User Management",
+    "userManagement": "Управление на потребители",
     "userUpdated": "User updated!",
     "username": "Username",
     "users": "Users"