[Stretch Goal] Add alert if processing historic data

Add a Shiny Alert which appears with a disclaimer whenever data are loaded from NWIS or a tab-delimited file and perception thresholds are auto-populated based on historical information in the NWIS database.

Disclaimer should state: "Perception thresholds have been automatically populated based on historic peak-flow data from NWIS. The user should carefully review these perception thresholds to verify they correctly represent historic peak streamflow."