XEP-0317: Hats, Update and complete the protocol

As a client developper the current Hats XEP is far from be usable and feature complete.

The goal of those changes is to:
- Allow clients to develop a nice UI to manage and assign hats, for example like Discord is doing (see https://support.discord.com/hc/en-us/articles/214836687-Discord-Roles-and-Permissions)
- Reconciliate existing implementations and their specificities, see https://docs.ejabberd.im/tutorials/muc-hats/
- Those changes were meant to be backward compatible with the current 0.2.0 version

This PR make the following changes:

- Specify a urn:xmpp:hats:commands:create command to add a hat to the available list
- Specify a urn:xmpp:hats:commands:destroy command to destroy a hat from the list
- Clarify how the service should broadcast the hat changes when it is edited, assigned, removed or destroyed
- Specify a way for an entity to get the complete list of hats using a hash in disco#info
- Add a hue optional parameter allowing entities to assign a color to the hat that can be displayed properly in any conditions on the client (as explained in XEP-0392: Consistent Color Generation)
- Standardize the all the form fields using XEP-0068
- Rename the :dlist, :don and :doff commands to :list, :assign and :unassign
- Add myself in the list of authors
- Fix some typos

Author: edhelas