DEV: extract inline js when baking theme fields (#6447)

* extract inline js when baking theme fields
* destroy javascript cache when destroying theme fields

This work is needed to support CSP work

spec/models/javascript_cache_spec.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+require 'rails_helper'
+
+RSpec.describe JavascriptCache, type: :model do
+  let(:theme) { Fabricate(:theme) }
+  let(:theme_field) { ThemeField.create!(theme: theme, target_id: 0, name: "header", value: "<a>html</a>") }
+
+  describe '#save' do
+    it 'updates the digest only if the content has changed' do
+      javascript_cache = JavascriptCache.create!(content: 'console.log("hello");', theme_field: theme_field)
+      expect(javascript_cache.digest).to_not be_empty
+
+      expect { javascript_cache.save! }.to_not change { javascript_cache.reload.digest }
+
+      expect do
+        javascript_cache.content = 'console.log("world");'
+        javascript_cache.save!
+      end.to change { javascript_cache.reload.digest }
+    end
+
+    it 'allows content to be empty, but not nil' do
+      javascript_cache = JavascriptCache.create!(content: 'console.log("hello");', theme_field: theme_field)
+
+      javascript_cache.content = ''
+      expect(javascript_cache.valid?).to eq(true)
+
+      javascript_cache.content = nil
+      expect(javascript_cache.valid?).to eq(false)
+      expect(javascript_cache.errors.details[:content]).to include(error: :empty)
+    end
+  end
+end