[IMP] mail, im_livechat: improve Discuss sidebar

The commit is to refactor the discuss sidebar

 - threads are now organized in categories based on the thread type e.g. chat, channel
 - categories can be folded or unfolded by clicking the category title
 - for active thread, even if the category is folded, it remains under the category title
 - for channel category, a new cog button is added to view all channels
 - the active indicator bar is removed. The active item now is highlighted with a different background color
 - thread avatar is used for livechat, chat and channel
 - for livechat and chat, threads are now sorted by last activity time (pin or message exchange)

closes odoo/odoo#70986

Task-id: 2440073
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>