Custom reporter effectiveness multipliers

Continuation of bug 10330 to remove hardcoded enum constants from the
Mantis codebase.

The summary reporter effectiveness statistics used to be generated by
using hardcoded multipliers for each different severity and resolution.
This posed a problem to anyone who defined their own custom severities
and resolutions, as there is no guarantee that the reporter
effectiveness would still remain a useful statistic.

This patch reintroduces the severity and resolution multipliers as user
customisable options in the Mantis configuration file. Users can now
correctly set the severity and resolution multipliers to custom values
and can define multipliers for custom severities and resolutions.