[Web] Add identity provider outage translation, remove unused needs_restart

data/web/lang/lang.en-gb.json

@@ -359,7 +359,6 @@
         "message": "Message",
         "message_size": "Message size",
         "nexthop": "Next hop",
-        "needs_restart": "needs restart",
         "no": "✕",
         "no_active_bans": "No active bans",
         "no_new_rows": "No further rows available",
@@ -561,6 +560,7 @@
         "goto_invalid": "Goto address %s is invalid",
         "ham_learn_error": "Ham learn error: %s",
         "iam_test_connection": "Connection failed",
+        "iam_unavailable": "The identity provider is not reachable, please try again later.",
         "imagick_exception": "Error: Imagick exception while reading image",
         "img_dimensions_exceeded": "Image exceeds the maximum image size",
         "img_invalid": "Cannot validate image file",