Make ThymeleafViewResolver @ConditionalOnWebApplication

If user creates a Thymeleaf app with a parent-child context then the
child should contain all the web-specific pieces (and they are likely
to fail fast if they need to be ServletContextAware, or slower if they
try to locate a WebApplicationContext at runtime). This can't happen
if the view resolver is being added to the parent.

Freemarker and Velocity already have similar tests because it is assumed
that they should be usable outside a web app, so this change just does the
same for Thymeleaf.

Fixes gh-1611