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.

assets/config.json.sample

@@ -2,6 +2,7 @@
   "jitsiLauncher": "server.domain.tld/subdir",
   "jitsiHost": "jit.si",
   "nextcloudBase": "https://cloud.domain.tld",
+  "legalNotice": "https://www.domain.tld/legal-notice",
   "allowAdhocMeetings": true,
   "launchTimeout": 2000,
   "backgroundImage": "path/to/backgroundimage",