Revert "Confirmation message opens in modal dialog"

This reverts commit f5caf6164d33c844e31f3ed3a7fdb199a93141c2. We
decided modals are unnecessary, instead we will use more prominent
alerts.
Use `:error` and `:success` for flash messages instead of alert so they can be appropriately styled
Move flash messages under nav and style as success, error or warning