"Test Now" button for alerts

[eradman]

In order to verify that Redash can send alerts, we often run through a series of manual steps:

1. Change the Refresh Schedule to a short interval (e.g. 1 minute), then wait
2. If no email is sent, change the alert condition (e.g. from < to >), then wait
3. If the alert was triggered, but not received, repeat step 2
4. If an alert was received, put the condition back and set the Refresh Schedule back to it's original value

This is complex because during setup we are trying to validate the query, the alert criteria, the configuration of Redash, and the ability of the destination to properly receive the alert. It would be far easier if we had a "Test Now" button.

[eradman]

@ezraodio1 proposed a solution to this in #7032