Use random password for CommunicateAsNewUser PBA

[shreyamalviya]

Merge this before #1433 to avoid messy merge conflicts!

---

What does this PR do?

#1410

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?

Testing Checklist

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

  Tested by running the PBA on Windows

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

[codecov]

Codecov Report

Merging #1434 (deb037c) into develop (54f80df) will increase coverage by 0.22%.
The diff coverage is 33.33%.

@@             Coverage Diff             @@
##           develop    #1434      +/-   ##
===========================================
+ Coverage    40.06%   40.29%   +0.22%     
===========================================
  Files          465      465              
  Lines        13835    13836       +1     
===========================================
+ Hits          5543     5575      +32     
+ Misses        8292     8261      -31     

Impacted Files	Coverage Δ
...key/post_breach/actions/communicate_as_new_user.py	66.66% <33.33%> (+66.66%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 54f80df...deb037c. Read the comment docs.