[IMP] im_livechat: don't auto-open on mobile

Currently on mobile, the livechat window would take all the viewport of
a small screen mobile browser. This combined with the "auto open"
feature can be desorienting: users could have only seen the "hello, how
may I help you ?" window and not know what to do or what is happening.

So this commit remove the "auto open" feature if user is on a too small
device (the user would thus have to click on "Have a Question").

closes #12148
opw-678012