FIX: Ensure theme_uploads_local only has one `/` at beginning (#17719)

Followup to c7dfb1c549

app/models/javascript_cache.rb

@@ -12,7 +12,7 @@ class JavascriptCache < ActiveRecord::Base
   end
 
   def local_url
-    "#{Discourse.base_path}/#{path}"
+    "#{Discourse.base_path}#{path}"
   end
 
   private

spec/models/theme_field_spec.rb

@@ -504,6 +504,7 @@ HTML
 
         expect(val["theme_uploads"]["test_js"]).to eq(js_field.upload.url)
         expect(val["theme_uploads_local"]["test_js"]).to eq(js_field.javascript_cache.local_url)
+        expect(val["theme_uploads_local"]["test_js"]).to start_with("/theme-javascripts/")
 
       end