Add a dialpad UI for PSTN lookup

Queries the homeserver for PSTN protocol support, and if found,
the add-room button on the DM rooms list section opens a context
menu instead with a 'dial pad' option as well as the current 'start chat'
dialog. Entering a number into this and pressing dial performs
a thirdparty user query for the given string and starts a DM with that
user.

res/css/views/rooms/_RoomList.scss

@@ -24,6 +24,9 @@ limitations under the License.
 .mx_RoomList_iconExplore::before {
     mask-image: url('$(res)/img/element-icons/roomlist/explore.svg');
 }
+.mx_RoomList_iconDialpad::before {
+    mask-image: url('$(res)/img/element-icons/roomlist/dialpad.svg');
+}
 
 .mx_RoomList_explorePrompt {
     margin: 4px 12px 4px;