[IMP] mail: load more messages: scrolling

- In the chat, when loading more messages, automatically scroll to the previous
oldest message instead of staying at the top;
- In the chat, don't use the event 'scrollTop < 50' to load more messages as
it won't work on mobile (all the webclient is scrolling, so the client action's
scrollTop will always be 0);
- Generalize the way to load more messages between chat and chatter by moving
the logic to the MailThreadMixin and creating a common template;
- Improve the style of this 'load more messages' button

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