Add functionality to update password with password hash (#15343)

* Add functionality to update password with password hash

This can be used to update a user's password with a direct password
hash instead of providing the password in plaintext.

* Use test helper for local mode

i18n/en.json

@@ -3038,6 +3038,10 @@
     "id": "api.user.update_password.oauth.app_error",
     "translation": "Update password failed because the user is logged in through an OAuth service."
   },
+  {
+    "id": "api.user.update_password.user_and_hashed.app_error",
+    "translation": "Only system administrators can set already-hashed passwords."
+  },
   {
     "id": "api.user.update_password.valid_account.app_error",
     "translation": "Update password failed because we couldn't find a valid account."