♻️ Add handling for Knapsack theme overrides

Prior to this commit, we were looking for themes yaml files relative to
the directory of the spawning script.  For Hyku that was always the
`Rails.root` directory.  However, when running specs in Knapsack, that
directory was `Knapsack::Engine.root`.

This unearthed a potential configuration issue; namely that we want
Knapsack's to control what themes are available, meaning we don't want
to require amending Hyku's themes.

So, we introduce a mechanism for looking up files first in the Knapsack
then in Hyku.

I discovered this bug in the specs for knapsack (below is the *Error stack trace*)

<details>
<summary>Error stack trace</summary>

```
2) Hyrax::Admin::AppearancesController with an administrator GET #show assigns the requested site as @site
     Failure/Error: get :show, params: {}

     Errno::ENOENT:
       No such file or directory @ rb_sysopen - config/home_themes.yml
     # /usr/local/bundle/gems/psych-3.3.4/lib/psych.rb:582:in `initialize'
     # /usr/local/bundle/gems/psych-3.3.4/lib/psych.rb:582:in `open'
     # /usr/local/bundle/gems/psych-3.3.4/lib/psych.rb:582:in `unsafe_load_file'
     # ./hyrax-webapp/app/controllers/hyrax/admin/appearances_controller.rb:19:in `show'
     # /usr/local/bundle/gems/rails-controller-testing-1.0.5/lib/rails/controller/testing/template_assertions.rb:62:in `process'
     # /usr/local/bundle/gems/devise-4.9.2/lib/devise/test/controller_helpers.rb:35:in `block in process'
     # /usr/local/bundle/gems/devise-4.9.2/lib/devise/test/controller_helpers.rb:104:in `catch'
     # /usr/local/bundle/gems/devise-4.9.2/lib/devise/test/controller_helpers.rb:104:in `_catch_warden'
     # /usr/local/bundle/gems/devise-4.9.2/lib/devise/test/controller_helpers.rb:35:in `process'
     # /usr/local/bundle/gems/rails-controller-testing-1.0.5/lib/rails/controller/testing/integration.rb:16:in `block (2 levels) in <module:Integration>'
     # ./spec/controllers/hyrax/hyrax/admin/appearances_controller_spec.rb:31:in `block (4 levels) in <top (required)>'
     # /usr/local/bundle/gems/webmock-3.19.1/lib/webmock/rspec.rb:39:in `block (2 levels) in <top (required)>'
     # ./hyrax-webapp/spec/support/multitenancy_metadata.rb:50:in `block (2 levels) in <top (required)>'
     # /usr/local/bundle/gems/rspec-retry-0.6.2/lib/rspec/retry.rb:124:in `block in run'
     # /usr/local/bundle/gems/rspec-retry-0.6.2/lib/rspec/retry.rb:110:in `loop'
     # /usr/local/bundle/gems/rspec-retry-0.6.2/lib/rspec/retry.rb:110:in `run'
     # /usr/local/bundle/gems/rspec-retry-0.6.2/lib/rspec_ext/rspec_ext.rb:12:in `run_with_retry'
     # /usr/local/bundle/gems/rspec-retry-0.6.2/lib/rspec/retry.rb:37:in `block (2 levels) in setup'
     # ./spec/spec_helper.rb:10:in `block (2 levels) in <top (required)>'
```

</details>

Related to:

- #2007
- #2008

The above two commits will require some reconciliation once this is incorporated.

app/controllers/hyrax/admin/appearances_controller.rb

@@ -16,8 +16,8 @@ def show
         add_breadcrumbs
         @form = form_class.new
         @fonts = [@form.headline_font, @form.body_font]
-        @home_theme_information = YAML.load_file('config/home_themes.yml')
-        @show_theme_information = YAML.load_file('config/show_themes.yml')
+        @home_theme_information = YAML.load_file(Hyku::Application.path_for('config/home_themes.yml'))
+        @show_theme_information = YAML.load_file(Hyku::Application.path_for('config/show_themes.yml'))
         @home_theme_names = load_home_theme_names
         @show_theme_names = load_show_theme_names
         @search_themes = load_search_themes

config/application.rb

@@ -37,6 +37,22 @@ class Application < Rails::Application
     # Add this line to load the lib folder first because we need
     config.autoload_paths.unshift("#{Rails.root}/lib")
 
+    ##
+    # @api public
+    #
+    # @param relative_path [String] lookup the relative paths first in the Knapsack then in Hyku.
+    #
+    # @return [String] the path to the file, favoring those found in the knapsack but falling back
+    #         to those in the Rails.root.
+    def self.path_for(relative_path)
+      if defined?(HykuKnapsack)
+        engine_path = HykuKnapsack::Engine.root.join(relative_path)
+        return engine_path.to_s if engine_path.exist?
+      end
+
+      Rails.root.join(relative_path).to_s
+    end
+
     # Settings in config/environments/* take precedence over those specified here.
     # Application configuration should go into files in config/initializers
     # -- all .rb files in that directory are automatically loaded.