ENH: Rename EMAIL_USER to EMAIL_USERNAME

[Miksus]

Renaming EMAIL_USER to EMAIL_USERNAME in app config.

[codecov-commenter]

Codecov Report

Merging #2 (40c1994) into master (b6c5315) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master        #2   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           47        52    +5     
=========================================
+ Hits            47        52    +5     

Impacted Files	Coverage Δ
flask_redmail/__init__.py	100.00% <100.00%> (ø)

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