[0.8.3] Set core min-version to 10.4

[phil-davis]

Because of

• Use rawurldecode for allowing "+" in guests emails #384 allow + in guest usernames
• Bump handlebars from 4.1.1 to 4.7.3 #393 bump handlebars 4.1.1 to 4.7.3 (which might not play nicely with core 10.3.2?)

See discussion around #400 (comment) to decide if we should actually do this.

[codecov]

Codecov Report

Merging #402 into release-0.8.3 will not change coverage by %.
The diff coverage is n/a.

@@               Coverage Diff                @@
##             release-0.8.3     #402   +/-   ##
================================================
  Coverage            17.85%   17.85%           
  Complexity              96       96           
================================================
  Files                   11       11           
  Lines                  532      532           
================================================
  Hits                    95       95           
  Misses                 437      437           

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