Make legal notice link configurable

The legal notice link in the navbar was hardcoded to the
bnvb website. Deployments without that legal notice showed
a dead link to a foreign site.

Move the URL into assets/config.json (legalNotice), like
the Nextcloud and adhoc buttons. The link stays hidden
unless the entry is configured.

launch.html

@@ -30,7 +30,7 @@
         </svg>
       </button>
       <div id="navActions" class="nav-actions">
-      <a class="button secondary" href="https://www.bnvb-ingenieure.de/impressum" target="_blank" rel="noopener noreferrer" data-i18n="legal_notice">
+      <a id="legalNoticeLink" class="button secondary hidden" href="#" target="_blank" rel="noopener noreferrer" data-i18n="legal_notice">
         Legal Notice
       </a>
       <a class="button secondary" href="https://git.bnvb.eu/stefan.tollkuehn/jitsi-launch" target="_blank" rel="noopener noreferrer" data-i18n="source">