Migrate Sortable.js to NPM provided version of Sortable

Summary

Sortable.js currently in use in several places like Issue Boards is static and is served from vendor/assets/javascripts and is currently at version 1.4.2, while the latest library version published on NPM is 1.7.0 (at the time of opening this issue).

Proposal

We can easily move the library from vendor to npm provided version to ensure that it always stays up to date and we can take advantage of Webpack's tree-shaking to load the package only on pages where it is being used.