When exposing group state, return is_openly_joinable
When exposing group state, return is_openly_joinable

as opposed to join_policy, which is really only pertinent to the
synapse implementation of the group server.

By doing this we keep the group server concept extensible by
allowing arbitrarily complex rules for deciding whether a group
is openly joinable.

matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse