Merge branch 'feat/group_saving' into 'master'

...u/idi/stud/savingsapp/controller/goal/GoalController.java

...a/no/ntnu/idi/stud/savingsapp/dto/goal/CreateGoalDTO.java

...va/no/ntnu/idi/stud/savingsapp/dto/goal/GroupUserDTO.java

...tud/savingsapp/exception/goal/GroupNotFoundException.java

...in/java/no/ntnu/idi/stud/savingsapp/model/goal/Group.java

.../ntnu/idi/stud/savingsapp/repository/GroupRepository.java

...java/no/ntnu/idi/stud/savingsapp/service/GoalService.java

...tnu/idi/stud/savingsapp/service/impl/GoalServiceImpl.java