Add a target to generate the gh-pages files.

It sets `github: true` for us.
Fixes #15096: Fix navbar forms in the non-responsive example
Bump Glyphicons to v1.9
Merge branch 'master' of github.com:twbs/bootstrap
Merge pull request #15134 from twbs/docs-gh

Add a target to generate the gh-pages files.
Update devDependencies.