Remove pending membership storage items and request and revoke membership calls

As discussed in the forum topic: https://forum.duniter.org/t/a-quoi-servent-les-pending-membership/11799, I would like to:

• remove request_membership call
• remove revoke_membership call
• remove PendingMembership storage item
• remove PendingMembershipsExpireOn storage item
• remove smith instance of membership pallet
• add add_member call in authority member pallet symmetrical to remove_member
• allow smith identities to change identity owner key
• make change_owner_key function of authority members pallet available as a call
• remove validate_identity call from identity pallet which is redundant with claim_membership of membership pallet

This would simplify a lot the protocol and remove potential bugs like the one targeted in #155 (closed) while keeping a good separation of concerns and make the event history easier to manage for the indexers.

Also linked to #136 (closed) in a way.