Settings UI updates

[brookewp]

Description

Some small changes to the design of the settings page, mainly in the header:

• replaced the Dropdown with the DropdownMenu component for improved a11y. Before, it wasn't possible to tab/use arrow keys to navigate within the popover as expected. Focus available on submenu now:

• replaced the deletion confirmation Modal with the ConfirmDialog component

There is also a back button in the header on the edit/add source pages. As a result, there are a few visual changes and a new form component added. The form will be handled in a follow-up, but it was added here so the header didn't look odd in the meantime:

After with form component:

Testing Instructions

• On the settings page, tab to the 'Add' menu button and use tab/arrow keys to navigate and select subitems
• Click on the delete icon to access confirmation dialog. Click 'cancel' and then open it again and click 'Confirm' to verify the behaviour is the same as before
• Edit/add source pages should behave the same as before, and have a back button in the header