A special Client only publishes Images in this Format

Fixes #8762

We use PubSub only on our account jid, this use case is what PEP
was made for. If PEP is discovered we know that certain PubSub
features are supported, see: https://xmpp.org/extensions/xep-0163.html#defaults

The current check for <feature var='http://jabber.org/protocol/pubsub'/>
is pretty useless, as it just tells us that there is a PubSub implementation
but not much about the features. Only `publish` and `subscribe` are MUST
in XEP-0060 which is not enough for our needs.

If there is ever need to discover a generic PubSub implementation
that is not PEP we should check for all the PubSub features we need
instead of only for <feature var='http://jabber.org/protocol/pubsub'/>

see: https://xmpp.org/extensions/xep-0163.html#support

Fixes #8765

Moved to the gajim-plugins repository

Fixes #8761

PEP supports a subset of PubSub and can also support publish options

- Add logging
- Move Node building code into own methods

Its for the Whiteboard Plugin

- Set application and menubar
- Remove deprecated settings

Fixes #8756

- Sort imports correctly (PEP8)

Avatar Refactoring

Closes #8743

See merge request !135

- Add support for Pubsub Avatars
- Dont poll for vCard Updates, only use XEP-0153
- Dont cache vCards
- Store the avatar SHA of roster contacts in the DB
- Store the current SHA of each contact in the Contacts Object
- Move some code into the ConnectionVcard Class

Update setup.py and .gitignore

See merge request !142

Fix deprecation warnings

See merge request !143