Bug 41600 - Remove old circuit display.

Bug 41600 - Remove extensions.torbutton.display_circuit since it will no
longer be used.

It also could not work as intended before because after disabling and
re-enabling, all the circuits and browser credentials that were
established whilst it was switched off would have been lost and were not
recovered.

Bug 41600 - We remove the additions for "controlCenter", i.e. the site
identity panel, since the tor circuit display no longer lies within it,
so our tour no longer uses it.

Bug 41600 - Change circuit display tour to use new button and panel.

This also fixes the "See My Path" button, which was broken before.

Bug 41600 - Add API to domain-isolator to get the stored SOCKS username
and password for a given domain and user context ID.

Also get rid of unused newCircuitForUserContextId.

Also remove the return statement from tor-control-port's watchEvent
since addNotificationCallback does not return a value.

Add cui-widget-panel class to the panel, which gives it the correct font.

fixup! Bug 31286: Implementation of bridge, proxy, and firewall settings in about:preferences#connection

Bug 41600 - Update the SVG fetch update script for fetching bridge
emojis and annotations, which we will also use for the flag emojis in
the tor circuit display.

We also:
+ Placed the bridgemoji files in a sub-directory.
+ Added a generation note to the generated files/directory.
+ Split the bridge emoji list into an external file, which we fetch in
  connectionPane.js so that the generating file is less fragile to
  a change in the javascript.
+ Store the locale emoji annotations using the string hex value as the
  key, rather than the emoji string sequence. This keeps it in line with
  the SVG file names, and removes ambiguity about whether the emoji
  character sequence includes the emoji variant selector U+FE0F or not.

Bug 41600 - Add new strings for the circuit display.