Fix broken radio buttons in alert dialog

[sarahd93]

The radio buttons for email alerts in the alert dialog were not working. The corresponding value from the model was a number but the dialog was expecting a string.

Fix parsing in the alert model to return a string as expected.

Checklist:

• Tests
• CHANGELOG Entry

[codecov]

Codecov Report

Merging #2326 into gsa-20.08 will decrease coverage by 0.00%.
The diff coverage is 66.66%.

@@              Coverage Diff              @@
##           gsa-20.08    #2326      +/-   ##
=============================================
- Coverage      52.76%   52.75%   -0.01%     
=============================================
  Files           1070     1070              
  Lines          25789    25791       +2     
  Branches        7312     7313       +1     
=============================================
- Hits           13607    13606       -1     
- Misses         11058    11061       +3     
  Partials        1124     1124              

Impacted Files	Coverage Δ
gsa/src/web/pages/cpes/details.js	11.11% <ø> (ø)
gsa/src/web/pages/extras/trashactions.js	5.49% <0.00%> (ø)
gsa/src/web/pages/hosts/dialog.js	50.00% <ø> (ø)
gsa/src/web/pages/reports/row.js	14.81% <ø> (ø)
gsa/src/web/routes.js	83.33% <ø> (ø)
gsa/src/gmp/models/alert.js	100.00% <100.00%> (ø)
gsa/src/web/entities/container.js	44.48% <0.00%> (-1.23%)	⬇️

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