Jekyll: exclude more files/folders.

We still can't exclude `./fonts/" and "./js/" due to https://github.com/jekyll/jekyll/issues/906
Fixes #10492 and some of #11970: round the proper corner on table cells in panels
Merge branch 'master' of github.com:twbs/bootstrap
grunt
Merge pull request #11978 from XhmikosR/clean-up-_gh-pages

Jekyll: exclude more files/folders.