OpenPGP for XMPP Pubsub (https://xmpp.org/extensions/inbox/pubsub-encryption.html,
currently a protoXEP) is implemented and activated when `encrypted` is set to `True` in
pubsub's `extra` data.
On item retrieval, the decryption is transparent if the key is known, except if the
`decrypt` key in `extra` is set to `False` (notably useful when one wants to checks that
data is well encrypted).
Methods and corresponding bridge methods have been implemented to manage shared secrets
(to share, revoke or rotate the secrets).
plugin XEP-0060's `XEP-0060_publish` trigger point as been move before actual publish so
item can be modified (here e2ee) by the triggers. A new `XEP-0060_items` trigger point has
also been added.
`encrypted` flag can be used with plugin XEP-0277's microblog data
rel 380
synchronisation used to be skipped when the node was existing and `resync` was not set,
however the case happens when a node is subscribed but not synchronised, thus this patch
update the behaviour to be sure to have the node synchronised.
rel 369
- cache only latest 20 items when disco infos are hidden and thus we can't discover
feature/RSM implementation
- better handling of args when calling `storage.getItems`
rel 365
