The JavaScript required to display the chart wasn't loaded on the admin
stats page.
We're not adding a test because we're going to move the budgets graph to
a different page on the pull request containing this commit.
Note we're changing the "Go back" link, since using a turbolinks refresh
broke this link when using the Chromium browser. Besides, there was an
inconsistency where some of the "Go back" links in admin stats pointed
to the admin stats page but other links pointed to `:back`.
TableSortable and Sortable javascripts were using the same CSS
class name to define completely different and separated
behaviours causing unexpected errors. Now `sortable.js` script
will use `.sortable` class and `table_sortable.js` will use
`.table-sortable` instead.
