Page for tablet/mobile browsers

The experience of ISN on pc browsers is good enough, but its layout on mobile and tablet browsers is all messed up.

This branch is used to develop a new page for tablets. Specifically,
0. Users using mobile browsers will be redirected to the tablet.html;
0. Theme and design should remain identical on the two versions, but layout may differ;
0. Try to maintain the concept of 'sequence' in the new mobile design.