Bug 31286: Implementation of bridge, proxy, and firewall settings in about:preferences#tor

This patch adds a new about:preferences#tor page which allows modifying
bridge, proxy, and firewall settings from within Tor Browser. All of the
functionality present in tor-launcher's Network Configuration panel is
present:

 - Setting built-in bridges
 - Requesting bridges from BridgeDB via moat
 - Using user-provided bridges
 - Configuring SOCKS4, SOCKS5, and HTTP/HTTPS proxies
 - Setting firewall ports
 - Viewing and Copying Tor's logs

In addition the following changes have been made:

 - The Networking Settings in General preferences has been removed
 - TorStrings has been removed from the SecurityLevel component and
   moved into a common shared module also used by about:preferences#tor