Refactor usage of service classes in MergeRequests namespace

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We currently have used services classes inside model methods. To get rid of this style violation, we'd like to refactor these. The refactoring should consider to move business logic to more appropriate places, e.g. presenters, facades or directly use the services, if no surrounding business logic exists at a place, where a service class was used inside a model's method.

For this ticket, to keep things small, we will consider the following places inside the MergeRequests namespace:

• app/models/merge_request.rb:266
• app/models/merge_request.rb:1233
• app/models/merge_request.rb:2117
• app/models/merge_request.rb:2196
• app/models/merge_request.rb:2322