Enhancements to the user subscriptions widget

When there are many products and subscriptions in APIM, it is not easy to filter them on the dev portal now. Users have to navigate through the whole list to find the one they need. If there could be a widget which can do the following, it would be very helpful.

- Accept product id as a parameter, such as from the query string of the URL etc., and use it to filter the products.
- Have a search box to allow users to search products by the name, description etc.
- Display the filtering result as links. When users click the link, it navigates to the product's page.

Start by locating the user subscriptions widget and its current product-list rendering in the developer portal. Review how product-page links and URL parameters are handled elsewhere, then define completion as supporting product-id filtering, text search, and links from each result to its product page.

Written by the indexing model from the issue text.