Add 3rdparty dependencies (used by libsignal)

Hopefully later we can use NPM for them, first want to get things to work

src/config.js

@@ -66,12 +66,18 @@ require.config({
         "xss":                      "node_modules/xss/dist/xss",
         "xss.noconflict":           "src/xss.noconflict",
 
+        // OMEMO/libsignal requirements
+        "Long":                     "3rdparty/long",
+        "protobuf":                 "3rdparty/protobuf",
+        "bytebuffer":               "3rdparty/bytebuffer",
+        "libsignal":                "3rdparty/libsignal-protocol-javascript/dist/libsignal-protocol",
+
         // Converse
         "converse":                 "src/converse",
 
         "converse-bookmarks":       "src/converse-bookmarks",
+        "converse-caps":            "src/converse-caps",
         "converse-chatboxes":       "src/converse-chatboxes",
-        "converse-caps":           "src/converse-caps",
         "converse-chatview":        "src/converse-chatview",
         "converse-controlbox":      "src/converse-controlbox",
         "converse-core":            "src/converse-core",
@@ -87,6 +93,7 @@ require.config({
         "converse-muc":             "src/converse-muc",
         "converse-muc-views":       "src/converse-muc-views",
         "converse-notification":    "src/converse-notification",
+        "converse-omemo":           "src/converse-omemo",
         "converse-otr":             "src/converse-otr",
         "converse-ping":            "src/converse-ping",
         "converse-profile":         "src/converse-profile",

src/converse-omemo.js

@@ -4,13 +4,13 @@
 // Copyright (c) 2013-2018, the Converse.js developers
 // Licensed under the Mozilla Public License (MPLv2)
 
-/*global libsignal */
 (function (root, factory) {
     define([
         "converse-core",
-        "tpl!toolbar_omemo"
+        "tpl!toolbar_omemo",
+        "libsignal"
     ], factory);
-}(this, function (converse, tpl_toolbar_omemo) {
+}(this, function (converse, tpl_toolbar_omemo, libsignal) {
 
     const { Backbone, Promise, Strophe, sizzle, $build, _, b64_sha1 } = converse.env;
 
@@ -25,10 +25,6 @@
 
     converse.plugins.add('converse-omemo', {
 
-        enabled (_converse) {
-            return !_.isNil(window.libsignal);
-        },
-
         overrides: {
             ChatBoxView:  {