(#125) notifications service implementation -- external events arrangement.

[SaintAngeLs]

Providing the notification service implementation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 33.63%. Comparing base (f9d98de) to head (5832a3f).
Report is 2 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #140   +/-   ##
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  Coverage   33.63%   33.63%           
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  Files          65       65           
  Lines         877      877           
  Branches       82       82           
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  Hits          295      295           
  Misses        560      560           
  Partials       22       22           

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[eggwhat]

Wouldn't it be easier/better to create notifications by using external events and their handlers? I.e. if we want to create a notification that someone has invited you to be your friend, utilize a FriendInvited event from friends service.