Convert GroupMemberStore to Vuex

Background

This is a follow-up of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8256#note_115517610:

GroupMemberStore should be refactored to use Vuex.

Implementation plan

This would be best handled in multiple MRs, an approach could be:

• Initialize the new Vuex store
  • Add a state.js file to ee/app/assets/javascripts/group_member_contributions/store
  • Include all the keys under this.state
  • Add an additional state key for the memberContributionsPath
• Add mutations and actions
  • Add a fetchContributedMembers action
  • Add an initialize action that sets initial state and the memberContributionsPath and should dispatch the fetchContributedMembers action
  • Add actions for setColumns, setMembers and sortMembers
  • Add related mutations for each action + mutation types
  • Add related tests for each mutation and each action
• Swap over to the new vuex store
  • Replace the existing store and dispatch the initialize action
  • Bind new vuex state + actions to the app.vue component

Note: Transforming the get methods would be optional, I'm not sure there's much more benefit than mapping to the state in this case