Reduce the number of "filters" used as helpers in Ruby templates

[jaredcwhite]

Currently we're including Bridgetown::Filters in the Bridgetown::RubyTemplateView::Helpers object. That brings over a whole bunch of methods which really only seem useful (and not always idiomatic Ruby) in a Liquid template. It would be advantageous to determine which filter methods are Liquid-specific and which are suitable for Ruby templates, and include only the suitable ones. This likely means we'll stop including Bridgetown::Filters directly and rather include some subset module, along with Liquid-only stuff getting moved elsewhere.