Automatic form mounting

[rockwellll]

This PR changes the default behavior for mounting forms.

Before

Form mounting was always manual, developers had to mount forms.

After

FormCollection automatically mounts forms into the page. Additionally, this feature can be disabled by passing a new configuration option autoMountForms to the initializer.

Hellotext.initialize('HELLOTEXT_BUSINESS_ID', { autoMountForms: false })

It discards the load event and instead uses the MutationObserver API to listen when forms are connected to the DOM, it ensures duplicate/already-loaded forms are not loaded again if a new form has been appended.