Respect validation_mode for external .herb templates

[marcoroth]

Follow up on #115.

When a gem ships a .html.herb template, it reaches ReActionView::Template::Handlers::Herb straight from the railtie. It never passes through Handlers::ERB, so nothing is in a position to rescue it.

#115 nonetheless forced validation_mode: :raise for every external template, including those, which turned what used to be a validation overlay into an uncaught exception on a template the application cannot edit:

ReActionView.config.validation_mode = :overlay
ReActionView.config.external_template_mode = :fallback

# /gems/some_gem/app/views/x.html.herb
# before: Herb::Engine::CompilationError
# after:  compiles, with the usual validation overlay

The mistake was putting the decision in Handlers::Herb, which cannot tell whether its caller will rescue. validation_mode_for is gone, and the mode is now passed in by the handler that owns the rescue.

Related #115