[FIX] mail, *: significantly reduce number of useless renders

* = hr_holidays, im_livechat, website_livechat

A huge amount of components where rendered for no reason, notably when typing a
new message (at every key press), which significantly slowed down the interface.

There were 2 causes to this number of renders:
- OWL rendering all children automatically, even if their props didn't change,
  unless `shouldUpdate` is overridden, which is now done.
- `useStore` selectors being way too generous in what they observed

task-2399731

closes odoo/odoo#62703

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

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