Notification refactorisation and wording

Acceptance Test

Please run bundle exec rails db:migrate as a migration has been add

To test, reset your database with bundle exec rails db:reset then

>> FactoryBot.build(:user).save  
>> Project.first.creator.add_role(:admin, Project.first)  
>> User.last.add_role(:admin, Challenge.first)  
>> Challenge.first.projects << Project.first  
>> ChallengesProject.first.pending!  
>> Notification.all  
>> ChallengesProject.first.accepted!  
>> Notification.last  
>> Challenge.first.projects.delete(Project.first)  
>> Notification.last

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Works on notification's email wording Refactorisation of challenges_project and challenge notifications

Closes Issue 340