Add autoprefixer-rails gem

https://github.com/ai/autoprefixer-rails
Merge pull request #420 from shinnn/autoprefixer-rails

fix #418 add autoprefixer-rails gem
fix #363 About page::create html
Merge pull request #424 from mozilla-japan/task363

fix #363 About page::create html
close #359 403::create html&css
close #362 503::create html&css
Merge pull request #425 from mozilla-japan/task359

close #359 403::create html&css
Merge pull request #426 from mozilla-japan/task362

close #362 503::create html&css
Improve style of modal window on /user/recipe/note