This commit moves the dial pad from a context menu to sit right by the explore button, in line with the designs in the linked issue. The dial pad button is only shown when PSTN support is detected.
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.
