Welcome message should use ADMINS set in environment and link to them

[bbelderbos]

This PR includes the changes of https://github.com/PyBites-Open-Source/karmabot/pull/96/files as I needed them here. I tested it in my env and admins are now used from the corresponding env variable and are properly linked:

[codecov-commenter]

Codecov Report

Merging #97 (a06ef62) into develop (0eaeefa) will increase coverage by 1.57%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop      #97      +/-   ##
===========================================
+ Coverage    54.01%   55.58%   +1.57%     
===========================================
  Files           23       23              
  Lines          735      743       +8     
  Branches        87       89       +2     
===========================================
+ Hits           397      413      +16     
+ Misses         318      312       -6     
+ Partials        20       18       -2     

Impacted Files	Coverage Δ
src/karmabot/commands/welcome.py	100.00% <100.00%> (+44.44%)	:arrow_up:
src/karmabot/settings.py	89.65% <100.00%> (+15.58%)	:arrow_up:

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[bbelderbos]

Thanks