debian: change severity mapping

[hdonnay]

Really seems like the severity mapping should be:

Debian Severity	Claircore Severity
unimportant	Negligible
low	Low
medium	Medium
high	High
*	Unknown

instead of

Debian Severity	Claircore Severity
unimportant	Low
low	Medium
medium	High
high	Critical
*	Unknown

Originally posted by @hdonnay in #1067 (comment)

[hdonnay]

@RTann, do you recall what the thinking was on the current mapping?

[crozzy]

#828 context here

[RTann]

Yeah #828 covers the reasoning. I aligned Debian's definitions with Red Hat's and that's how the severity mapping was done. I think it makes sense, as users who may want to filter by critical vulns will never see Critical Debian's vulns if we never use it