fix(checkbox): fix disable state with reactive forms

[mrchops1024]

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• I read and followed the CONTRIBUTING.md guide.
• I read and followed the New Feature Checklist guide.

Short description of what this resolves:

When using nb-checkbox with reactive forms, disabling the form control does not disable the underlying input. Implementing the setDisabledState method fixes this issue. See #296 for the initial report.

[codecov]

Codecov Report

Merging #599 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #599      +/-   ##
=========================================
+ Coverage    70.5%   70.6%   +0.09%     
=========================================
  Files         124     124              
  Lines        3590    3592       +2     
  Branches      260     260              
=========================================
+ Hits         2531    2536       +5     
+ Misses       1010    1007       -3     
  Partials       49      49

Impacted Files	Coverage Δ
...rk/theme/components/checkbox/checkbox.component.ts	93.93% <100%> (+10.06%)	⬆️

[nnixaa]

@mrchops1024 cool, thanks! 👍