FIX: Use theme screenshot names in theme fields (#31852)

Currently we allow for 2 theme screenshots to be specified,
with a lightweight spec to allow both a light and dark
version of the screenshot. However, we were not storing this
screenshot name anywhere, so we would not be able to use it
for light/dark switching.

This commit fixes that issue, and also does some general refactoring
around theme screenshots, and adds more tests.

lib/compression/strategy.rb

@@ -38,18 +38,6 @@ module Compression
       Pathname.new(filename).realpath.to_s
     end
 
-    # https://guides.rubyonrails.org/security.html#file-uploads
-    def sanitize_filename(filename)
-      filename.strip.tap do |name|
-        # NOTE: File.basename doesn't work right with Windows paths on Unix
-        # get only the filename, not the whole path
-        name.sub! %r{\A.*(\\|/)}, ""
-        # Finally, replace all non alphanumeric, underscore
-        # or periods with underscore
-        name.gsub! /[^\w\.\-]/, "_"
-      end
-    end
-
     def calculate_available_size(max_size)
       1024**2 * (max_size / 1.049) # Mb to Mib
     end

lib/compression/tar.rb

@@ -9,7 +9,7 @@ module Compression
     end
 
     def compress(path, target_name)
-      tar_filename = sanitize_filename("#{target_name}.tar")
+      tar_filename = FileHelper.sanitize_filename("#{target_name}.tar")
       Discourse::Utils.execute_command(
         "tar",
         "--create",