Use Sassy CSS and move Sass variables to global config file

Since we are about to add a loading screen for the EcosystemGrid that
will share some styles with that component, it makes sense to convert
our 'application.css' to a Sassy CSS file and use variables to share the
style values.
Add autoprefixer-rails gem

Since I'm going to add an animation to the grid loading screen (and more
CSS animations in the future), autoprefixer will really DRY up the
styles.
Add GridLoadingScreen component

The only reason to make this it's own component was for the sake of
organization, following the single responsability principle. The
LifeApp's only responsiblity is to hold all the pieces of the page and
pass them the right data - it shouldn't contain the logic or
presentation for any parts of the screen within itself.