UI for **Add app to PWA store** feature

[cherukumilli]

• create a new element for processing / adding a new app URL
• Add paper-input field for the user to enter the app's URL
• Enter URL text field should be prefixed by https://
• Add a + paper-button next to the url input field
• If the user doesn't enter a protocol along with the url then append the https:// to the hostname
• If the user enters an invalid URL then display error