Add param to set whether to save on reorder

[pascallaliberte]

Co-dependent PRs:

• https://github.com/bullet-train-co/bullet_train-base/pull/80
• (A PR in bullet_train should be added for tests)

This PR addresses the changes to the Stimulus controller:

• [X] Add the saveOnReorder boolean value, defaulting to true

Note: this PR is on top of unmerged PR #6, which introduces the Stimulus controller. The merge target branch is add-sortable-stimulus-controller for that reason.