Merge branch 'feature/join-group' into 'master'

lib/mobilizon/actors/actor.ex

lib/mobilizon/actors/actors.ex

lib/mobilizon/actors/member.ex

lib/mobilizon/events/events.ex

lib/mobilizon_web/resolvers/event.ex

lib/mobilizon_web/resolvers/group.ex

lib/mobilizon_web/schema.ex

lib/mobilizon_web/schema/actors/member.ex

...po/migrations/20190301141830_move_member_role_to_enum.exs

priv/repo/migrations/20190301143831_actor_group_openness.exs

test/mobilizon/actors/actors_test.exs

test/mobilizon_web/resolvers/group_resolver_test.exs

test/mobilizon_web/resolvers/member_resolver_test.exs

test/support/factory.ex