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/templates/livechat-external-login-modal.js

@@ -0,0 +1,56 @@
+import { __ } from 'i18n'
+import { html } from 'lit'
+
+export const tplExternalLoginModal = (el, o) => {
+  // eslint-disable-next-line no-undef
+  const i18nRemotePeertube = __(LOC_login_remote_peertube)
+  // eslint-disable-next-line no-undef
+  const i18nRemotePeertubeUrl = __(LOC_login_remote_peertube_url)
+  const i18nRemotePeertubeOpen = __('OK')
+  return html`<div class="modal-body livechat-external-login-modal">
+    <form class="converse-form chatroom-form" @submit=${(ev) => el.openRemotePeertube(ev)}>
+      <label>
+        ${i18nRemotePeertube}
+        <input
+          type="url"
+          placeholder="${i18nRemotePeertubeUrl}"
+          class="form-control ${o.remote_peertube_alert_message ? 'is-invalid' : ''}"
+          name="peertube_url"
+          ?disabled=${o.remote_peertube_state === 'loading'}
+        />
+      </label>
+      <input
+        type="submit"
+        class="btn btn-primary"
+        value="${i18nRemotePeertubeOpen}"
+        @keyup=${el.onKeyUp}
+        ?disabled=${o.remote_peertube_state === 'loading'}
+      />
+      ${
+        o.remote_peertube_state !== 'loading'
+          ? ''
+          : html`<small class="form-text text-muted">${
+            // eslint-disable-next-line no-undef
+            __(LOC_login_remote_peertube_searching)
+          }</small>`
+      }
+      ${!o.remote_peertube_alert_message
+        ? ''
+        : html`<div class="invalid-feedback d-block">${o.remote_peertube_alert_message}</div>`
+      }
+      ${!o.remote_peertube_try_anyway_url
+        ? ''
+        : html`<div class="form-text">
+          ${
+            // eslint-disable-next-line no-undef
+            __(LOC_login_remote_peertube_video_not_found_try_anyway)
+          }
+          <button class="btn btn-primary" onclick="window.location.href='${o.remote_peertube_try_anyway_url}'">${
+            // eslint-disable-next-line no-undef
+            __(LOC_login_remote_peertube_video_not_found_try_anyway_button)
+          }</button>
+        </div>`
+      }
+    </fieldset>
+  </form></div>`
+}