MDL-36316 useragent: Reduce user agent sniffing

* core_useragent: Introduce is_vendor methods to improve readability
  when we are just detecting if a browser is from a vendor, rather than
  the exact version

* Remove uncessary browser version checks when we are just detecting
  vendors - this makes the intention of our sniffing clearer.

* Remove sniffing for browsers which we do not support, grades/ajax/tinymce
  all support modern browsers so there is no need to sniff for them.