#5209366 Reader Lists: Add item adder component

Changes proposed in this Pull Request

• Adds a new ItemAdder component for adding new sites to a list. This component consists of three pieces:
  1. A text input for receiving the user’s query.
  2. A add/remove toggle button which appears if the query is recognized as a URL.
  3. Search results rendered as cards; each card can be added to the list via a CTA button.

Testing instructions

1. Navigate to a Reader List of your choice and navigate to the Sites tab.

2. Enter a query into the search input. Ensure that search results appear below the input.

3. Add some of the search result feeds into the reader list. Ensure that they are shown in the Added sites section.

4. Remove sites from the Added sites section. Ensure that a confirmation dialogue is shown before processing the removal. Ensure that the site is removed from the list by dismissing the dialogue with the Remove button.

5. Enter an RSS feed address to the search input. Ensure that a button appears below the input for following the feed.

6. Click on the button and ensure that the feed gets added to the list. Clicking the button again will remove the feed from the list.