When prepending or appending view paths, Rails will create an ActionView::Resolver
for each String or Pathname argument. Previously, these were not cached. If view
paths were added in a request context (which is a common thing in a multi-tenant
application), new resolvers were created on each request. This effectively
disabled template caching for the added view paths.

Further, under certain circumstances, it also created a memory leak.

This commit fixes both issues by turning dynamic view path strings into cached
resolvers.

Co-authored-by: Dominik Schöler <dominik.schoeler@makandra.de>
Co-authored-by: John Hawthorn <john@hawthorn.email>