Fixed flickering and simplified calculations

- Removed offending line causing a flicker to the default top state
- Simplified calculation of the top offset when in the bottom state
Rename and organize nav partials for cleaner file structure
correct event naming of `scroll` and `load' event

closes #12337
Fix #12375: stop using document.body.scrollTop as it's deprecated in the strict mode

Closes #12377
Merge pull request #12862 from sultano/patch-1

Fixed flickering and simplified calculations
Fix #12845 - Modal#hide should only be called with an event argument

Closes #12850
Merge branch 'hnrch02-modal-toggle'
move sauce_browsers.yml into /grunt/; fixes #13065
Merge branch 'master' of github.com:twbs/bootstrap
Update gruntfile to point to new customizer nav, run grunt
grunt