LP#1776757: (follow-up) keep input for amount as type "number"

LP#1776757: (follow-up) keep input for amount as type "number"

Doing this preserves the form validation that ensures that
the user cannot submit the form with a non-numeric amount.

This does lose forcing the display of default amounts such as
"1.50" to two decimal places, but I'm figuring that form validation
takes priority pending a longer-term solution such as a custom
value validator or using ng-currency.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>