Allow adding participants by username in `RecruitmentService` (#459)

[xelahalo]

Closes #459

[Whathecode]

@xelahalo Thanks a lot! Looks good to go now.

Made some changes to your PR:

• Added a cleaner solution to allow for application service method overloading.
• Added a helper method to change JSON class discriminators during migration.
• Fixed studies README which still documented the non-overloaded application service methods.
• Squashed commits which simply undid prior commits (and thus don't contribute to a meaningful commit history). Also, it should be a goal for each commit to compile and pass tests.
• More self-documenting code here and there and various style fixes more in line with my style. :)