Replace with asset pipeline

[kenhys]

Describe the bug

With netlify migration, now we use middleman as framework.
In the previous sinatra application (Heroku), we used customized version of
sinatra-asset-pipeline.

Now for migration simplicity, we don't use asset pipeline yet.

Link to the problematic contents

It still use non-compressed, separated js or css.

• source/layouts/layout.erb
• source/layotus/minimal_layout.erb

Expected explanation

Introduce asset pipeline and restructure assets directory.

Additional context

No response