Allow login with email address

[imnasnainaec]

Resolves #3608

Also add more safeguards against collision of one user's username with another user's email address.

Acceptance tests:

• Create a user (A) with a username different than the email address

  • Log in with the username
  • Log in with the email address
  • Try to create a second user (B) with username that matches the email address of the first user (A)
    • See error message: Username not available.

• Create a user (C) with a valid email address as the username and a fake email address for the email address

  • Try to create a user (D) with email address that matches the username of another user (C)
    • See error message: Email address not available.
  • Log in to a user with an email address for a username (C) and in User Settings, update the email address to match the username

---

This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 76.19048% with 5 lines in your changes missing coverage. Please review.

Project coverage is 73.26%. Comparing base (e663ff9) to head (70151e7).
Report is 22 commits behind head on master.

Files with missing lines	Patch %	Lines
Backend/Controllers/UserController.cs	66.66%	3 Missing and 1 partial ⚠️
Backend/Services/PermissionService.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3673   +/-   ##
=======================================
  Coverage   73.26%   73.26%           
=======================================
  Files         285      285           
  Lines       10627    10627           
  Branches     1322     1321    -1     
=======================================
  Hits         7786     7786           
  Misses       2451     2451           
  Partials      390      390           

Flag	Coverage Δ
backend	82.81% <66.66%> (+<0.01%)	⬆️
frontend	65.78% <100.00%> (-0.01%)	⬇️

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[jasonleenaylor]

Reviewed 11 of 14 files at r1, 9 of 9 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @imnasnainaec)