Warning about invisible connections/variable in the Web UI

Hello,

Currently, you can define a new connection/variable in several ways.
* Metabase
* Environment variable
* Secret backend

However, only the connections defined with the Metabase are visible from the Web UI. This is surprising among users because a natural way to check connection/variable configuration is to use the Web UI, which may not show complete information.
I would be happy if there was a warning stating that not all connections / variables are visible when the user has a configured secret backend or when the `AIRFLOW__CONN_*` /`AIRFLOW__VAR_*` environment variable is defined. This should appear above the list of connections/variables.

This change is intended to improve the user experience.

Best regards,
Kamil Breguła

Start at the Web UI pages that list connections and variables, and trace how those lists are assembled. Add an explanatory warning above each list when environment-variable or secret-backend configuration may be hidden; done means users can see that the displayed configuration is incomplete in those cases.

Written by the indexing model from the issue text.