Allow project group links to be expired

app/assets/javascripts/dispatcher.js

app/controllers/projects/group_links_controller.rb

app/helpers/members_helper.rb

app/models/concerns/expirable.rb

app/models/member.rb

app/models/project_group_link.rb

app/views/projects/group_links/index.html.haml

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

app/workers/remove_expired_group_links_worker.rb

config/initializers/1_settings.rb

...e/20160818205718_add_expires_at_to_project_group_links.rb

db/schema.rb

doc/workflow/share_projects_with_other_groups.md

spec/features/projects/group_links_spec.rb

spec/workers/remove_expired_group_links_worker_spec.rb

spec/workers/remove_expired_members_worker_spec.rb