provide a mechanism to not apply the taxcalculator rules for specific financial types

Sometimes, a site with this extension on it would like to ignore these rules for a specific financial type (e.g. events that are taxed based on the location of the event, not the location of the person registering).

This PR enables that with:

1. An additional "ignore" setting per financial type.
2. "ignored" financial types are filtered out of the function getTaxableFinancialTypes
3. A pre-filtering of the buildForm hook implementation that skips it for forms with no taxable financial types.