Fix release build job

[leoarnold]

The CSS code for the flipflop web interface is generated by the rake task assets:compile, hence it should not be committed to the repository at all, because its content depends on the actual versions of gems activated at build time. We therefore remove it and add it to .gitignore.

Furthermore, in #40 the stylesheet file was renamed from _flipflop.css to _flipflop.html.erb which is problematic because

• the file is not HTML
• the file is not a template
• the filename in rake assets:compile was not adjusted, so the generated content is now ignored and instead the committed content is used.

We revert the renaming of the file and use render(file: ... to insert its contents into the view, thereby fixing all issues.