Initialize severity value with 0 in powerfilter

[swaterkamp]

What:

• Initialize severity value in the powerfilter's SeverityValuesGroup with 0.
• Use severity as filter keyword for result filter instead of cvss_base

Why:

• The 0 was the default value of the NumberField, which was displayed, but was not stored in the dialog state to be
  sent with the filter update.
• Using severity is more consistent with other entities' filters

How:
Manuel tests and new automatic test added

Checklist:

• Tests
• CHANGELOG Entry
• Labels for ports to other branches

[codecov]

Codecov Report

Merging #3031 (b12f793) into gsa-20.08 (5363085) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           gsa-20.08    #3031   +/-   ##
==========================================
  Coverage      53.74%   53.74%           
==========================================
  Files           1073     1073           
  Lines          25912    25912           
  Branches        7375     7375           
==========================================
  Hits           13926    13926           
  Misses         10882    10882           
  Partials        1104     1104           

Impacted Files	Coverage Δ
gsa/src/web/pages/results/filterdialog.js	37.50% <ø> (ø)
.../web/components/powerfilter/severityvaluesgroup.js	91.66% <100.00%> (ø)

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