Improve tawk button position

Tawk button is overlapping the logout button in the backoffice on mobile

Currently tawk's button is in the most top layer using `z-index` `!important`.
Seems like there is no way to configure this. The only way is to hide it and use tawk API to show / hide the chat.

https://help.tawk.to/article/using-a-text-link-or-button

UI-wise, we can use their image for the show / hide button so it basically will look the same.