Add expiration date to group memberships

app/assets/javascripts/dispatcher.js

app/controllers/groups/group_members_controller.rb

app/models/group.rb

app/models/project.rb

app/models/project_team.rb

app/views/groups/group_members/_new_group_member.html.haml

app/views/groups/group_members/update.js.haml

app/views/shared/members/_member.html.haml

features/steps/group/members.rb

features/steps/project/team_management.rb

lib/api/entities.rb

lib/api/members.rb

...s/members/master_adds_member_with_expiration_date_spec.rb

spec/requests/api/members_spec.rb