Open the <dialog> with a <button>

Nest the page’s content within a <main> element, and render a <dialog> element as its sibling, which we’re polyfilling with Google’s dialog-polyfill package.

Move the Task selection elements from the tasks#index template to a new events#new template, and render it in-line within the <dialog> element.

In its place, render a set of <button> elements that will open the <dialog> to further manage Bulk Action selection.

In order to manage that <dialog> element interaction, this commit introduces the ControlsDialog and Dialog Stimulus controllers to work in tandem.