perf(ngForm,ngModel): move initial addClass to the compile phase

Other then moving from link to compile time the only difference (which unit tests didn't seem to care about) is that `$animate.addClass(VALID_CLASS)` and `$animate.removeClass(INVALID_CLASS)` are no longer called at link time since VALID_CLASS is added at compile and INVALID_CLASS shouldn't exist. If anyone depends on the initial animation calls or on the `.removeClass(INVALID_CLASS)` then this change won't work, at least in its current form...
Performance wise I assume the extra string concat here is actually better because each addClass will do split+forEach+regex+get/setAttribute. But this is compile time so I don't think it matters much, and calling `addClass` twice looks more readable so I'll go ahead with that for now...