Pause/delay NTIE refresh on certain conditions

[podliashanyk]

Per now NTIE refresh endpoint is polled (via HTMX attr) every REFRESH_INTERVAL seconds regardless of the state of the app/zino server.
NTIE refresh should not be performed:

• While table (or events page in general) is already loading, i.e. get_events is running, and load is not complete yet. Otherwise the table is loaded OK, but an error is raised. Issue: Ensure NTIE refresh requests never start before get_events has completed #144
• While connection to zino server is established to be down by Howitz. Otherwise a new error alert with the same error message appears in the UI every REFRESH_INTERVAL seconds.