JS/IE8: fix IE8 error caused by focus()

No more error when using the jQuery wrapper. Focus handling is pretty
outdated, needs special care as soon as we have auto-refreshing search
fields.
JS/IE: remove unused conditional comments

There is no IE7 support and no special handling for IE9, therefore
I removed unused conditional comments. Left IE8 there as it might ask
for special CSS - however it doesn't right now.

Also added missing iframe-class for Iframes on IE8.
JS/IE8: deliver legacy jQuery for IE8

This patch makes IcingaWeb deliver a legacy jQuery version for IE8
as it is no longer supported in jQuery 2.x. JS for IE8 will not be
delivered minified to ease troubleshooting on that buggy platform.

fixes #5866
refs #6417