re-bundle for controller rename later
rename spreadsheet to sheet
add devise views for editing later
let nav has same tone on all links
use handsontable for interactive spreadsheet

studying in individul sheet show
to precompile all *.js files

default, vendor/assets/javascript/*.js will not be precompiled.
in this way to ensure all *.js files be precompiled.
to precompile all *.css files also

default, vendor/assets/stylesheet/*.css won't be precompiled
in this way to ensure all *.css files be precompiled.
include all assets in application

precompile all vendor assets will failed on heroku site.
for now, put all assets onto application.
yet, this will cause unncessary assets loading on certain pages.