Split loadList() into fetchMilestones() and fetchAssignees() in issue boards store

The following discussion from !13203 (merged) should be addressed:

• @winh started a discussion: (+1 comment)

  I think instead of having a generic loadList(), it makes sense to have fetchMilestones() and fetchAssignees() / fetchUsers(). That would

  • make it easier to understand what data is fetched here.
  • avoid listPath and listType props
  • remove logic like if (this.listType === 'milestones') below