PWA App

[Spuds]

All of the initial items to provide a basic functional PWA .. includes manifest generation and new icons

[codecov]

Codecov Report

Attention: Patch coverage is 50.50000% with 99 lines in your changes are missing coverage. Please review.

Project coverage is 32.90%. Comparing base (9552e90) to head (6d9ab46).
Report is 1 commits behind head on development.

Files	Patch %	Lines
sources/ElkArte/AdminController/ManageFeatures.php	0.00%	64 Missing ⚠️
sources/ElkArte/Themes/Theme.php	31.25%	22 Missing ⚠️
sources/Subs.php	22.22%	7 Missing ⚠️
sources/ElkArte/Controller/Offline.php	0.00%	6 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                @@
##             development    #3775      +/-   ##
=================================================
+ Coverage          32.81%   32.90%   +0.09%     
- Complexity         12889    12925      +36     
=================================================
  Files                286      288       +2     
  Lines              62419    62607     +188     
=================================================
+ Hits               20484    20603     +119     
- Misses             41935    42004      +69     

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