[REF] mail: code refactoring

- extended_chat_window feature in its own files: as it is only available in the
  backend, we don't want it in the frontend assets (which contain the 'basic'
  chat_window)
- move followers feature in its own files
- creation of the 'mail.utils' module in utils.js
- creation of systray.less to define the style of the mail item and dropdown
  in the navbar
- use of chat_manager's search_partner function in PartnerInviteDialog
- some code linting

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