luci-base: harmonize JS class naming and requesting

 - Make builtin classes available via `require` to allow view code to
   request external and internal classes in a consistent manner without
   having to know which classes are builtin and which not

 - Make base classes request any used class explicitely instead of
   relying on implicitly set up L.{dom,view,Poll,Request,Class} aliases

 - Consistently convert class names to lower case in JSdoc to match
   the names used in `require` statements

 - Deprecate L.{dom,view,Poll,Request,Class} aliases

Signed-off-by: Jo-Philipp Wich <jo@mein.io>