Make email matching case insensitive for external invites

[knolleary]

Fixes #4781

This updates how we lookup invites for external users to be case insensitive on email.

Existing test updated to cover the scenario.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.79%. Comparing base (e173733) to head (34823a1).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
forge/db/controllers/Invitation.js	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4783   +/-   ##
=======================================
  Coverage   78.79%   78.79%           
=======================================
  Files         311      311           
  Lines       14782    14782           
  Branches     3385     3385           
=======================================
  Hits        11647    11647           
  Misses       3135     3135           

Flag	Coverage Δ
backend	78.79% <66.66%> (ø)

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[hardillb]

Will merge when test finish