Log in with external Peertube account (#348):

* For anonymous users: new "log in using an external account" dialog, with the "remote Peertube account" options
* ConverseJS: using global vars for custom localized string (injected
  using Webpack)

conversejs/custom/shared/modals/livechat-external-login.js

@@ -0,0 +1,20 @@
+import { __ } from 'i18n'
+import BaseModal from 'plugins/modal/modal.js'
+import { api } from '@converse/headless/core'
+import { html } from 'lit'
+import 'livechat-external-login-content.js'
+
+class ExternalLoginModal extends BaseModal {
+  remotePeertubeError = ''
+
+  renderModal () {
+    return html`<converse-livechat-external-login-content></converse-livechat-external-login-content>`
+  }
+
+  getModalTitle () {
+    // eslint-disable-next-line no-undef
+    return __(LOC_login_using_external_account)
+  }
+}
+
+api.elements.define('converse-livechat-external-login', ExternalLoginModal)