New API endpoint for registration status

[shreyamalviya]

What does this PR do?

Fixes a part of #2105

PR Checklist

• Have you added an explanation of what your changes do and why you'd like to include them?
• Is the TravisCI build passing?
• Was the CHANGELOG.md updated to reflect the changes?
• Was the documentation framework updated to reflect the changes?
• Have you checked that you haven't introduced any duplicate code?

Testing Checklist

• Added relevant unit tests?
• Have you successfully tested your changes locally? Elaborate:

  Tested by running locally

• If applicable, add screenshots or log transcripts of the feature working

[codecov]

Codecov Report

Merging #2149 (4801235) into develop (9442b66) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #2149      +/-   ##
===========================================
+ Coverage    56.91%   56.94%   +0.02%     
===========================================
  Files          488      489       +1     
  Lines        13274    13282       +8     
===========================================
+ Hits          7555     7563       +8     
  Misses        5719     5719              

Impacted Files	Coverage Δ
monkey/monkey_island/cc/resources/auth/register.py	100.00% <ø> (ø)
monkey/monkey_island/cc/app.py	80.53% <100.00%> (+0.26%)	⬆️
...ey_island/cc/resources/auth/registration_status.py	100.00% <100.00%> (ø)

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