Define "soft" and USB WebAuthn development options

In order to allow and simplify the development and testing of
WebAuthn [1] based apps and websites, the security options must be set
to allow "soft tokens" [1].

Also see the section about requirements for testing [3] of the Mozilla
"Security/QA/TestPlans/Web Authentication" wiki page for more details.

[1]: https://webauthn.io
[2]: https://webauthn.bin.coffee/ff58
[3]: https://wiki.mozilla.org/Security/QA/TestPlans/Web_Authentication#Requirements_for_testing

GH-226

snowblocks/firefox/user.js

@@ -481,6 +481,11 @@ user_pref("security.default_personal_cert", "Ask Every Time");
  */
 user_pref("security.OCSP.enabled", 1);
 
+
+/* Allows to toggle "soft tokens" to develop and test WebAuthn. */
+user_pref("security.webauth.webauthn_enable_softtoken", false);
+user_pref("security.webauth.webauthn_enable_usbtoken", false);
+
 /*
  * Always ask to save logins and passwords for websites.
  * Available within the UI under: