Add controls for adding websites and deleting sites and fields in edit mode

Also replace the 'Go Back' icon with one for canceling edits
and returning to view mode when editing an existing item
Prefill URL with current tab's URL when adding a new website

When adding a new URL field to an existing item,
prefill with the current tab's URL when using
the browser extension.

This simplifies adding another site for a login which is used
across domains.
Fix error if button is unmounted less than 1s after mount
Animate drop-down menu opening
Fix items being duplicated on save

Ensure the temporary cloned item created for editing
has the same ID as the original item.
Add item actions for moving to / restoring from trash