Display a static Dialpad

This PR displays a static image of a dialpad. The "click to call" feature is already implemented in master.

It should be merged once https://github.com/mozilla/talkilla/pull/381 have landed.

Related card: https://trello.com/c/9pqhFG9J/359-1-as-a-user-having-clicked-on-the-phone-icon-in-the-panel-i-want-to-use-a-dialpad-so-i-dial-numbers-out