Added PSTN dialer in the sidebar

Related card: https://trello.com/c/ApAYdelh/329-2-as-a-user-i-can-dial-any-pstn-number-if-an-spa-i-ve-added-allows-it

This PR adds a PSTN (landphone) number dialer to the sidebar. Note that the Talkilla SPA doesn't support the `pstn-call` capability, so you have to enable it manually if you want to test this feature live.