Remove dependency on javax.servlet

This is a breaking change but with limited impact:
 * `Context.getHttpServletRequest` / `Response` methods were `@Deprecated` already
 * `FlashCookie` / `SessionCookie`'s `save` method now take `Result` as a second parameter
 * several classes relocated (`ninja.async` → `ninja.servlet.async`, `ninja.utils.CookieHelper` → `cookie.servlet.CookieHelper`)

This paves the way for supporting other containers, such as Netty or Grizzly.

Maybe `ninja.servlet` should be moved to its own `ninja-servlet` module so it's really optional, but `javax.servlet` is _provided_ already so it's not really an issue in practice, only an _overhead_ of a handful classes.