- Implements new format "Copyright 2011-xxxx Twitter, Inc.", which resonates well with our History page and more accurately reflects the copyright status of the repo
- grunt dist to update all the files

Update Gruntfile.js to copy 'dist/' files to 'docs/dist/' with 'grunt dist' task; Fixes #12030: navbar toggle focus state

* Since we no longer have `.nav-list`, I've nuked the `.nav-header`
* However, dropdowns can still have section headings, so I've kept that
around with the addition of `.dropdown-header`
* Restyled it a smidge as well