Upgrade to Hugo 0.16x and zen theme v7.1.0

- Bump themes/zen from v5.7.0 to v7.1.0 (vanilla CSS, no libsass).
- Move layouts to Hugo's new template structure (_default -> root,
  index.html -> home.html, shortcodes -> _shortcodes).
- Replace deprecated language API in templates (Locale, Direction, Name)
  and sync base templates and mobile menu with theme v7.
- Config: languageName/languageCode -> label/locale; move copyright to
  the top-level key the v7 footer reads.
- Port _custom.scss to assets/css/_custom.css and map page/body
  backgrounds to v7's --color-body-bg/--color-page-bg.
- CI: build with hugomods/hugo:0.165.0.

hugo.yaml

@@ -1,16 +1,15 @@
 baseURL: 'https://hackertours.hamburg.ccc.de/'
 defaultContentLanguage: de
 defaultContentLanguageInSubdir: true
-#languageCode: 'de-de'
 title: 'Hamburg Hackertours'
 theme: "zen"
+copyright: "[CCC Hansestadt Hamburg e.V.](https://hamburg.ccc.de) ist nur der Vermittler der Touren. Für das Angebot und die Durchführung der Touren sind die jeweiligen Veranstalter verantwortlich."
 ignoreLogs:
   - 'warning-frontmatter-params-overrides'
 
 #https://themes.gohugo.io/themes/hugo-theme-zen/#configuration
 params:
     contact: "ticket@hackertours.hamburg.ccc.de"
-    copyright: "[CCC Hansestadt Hamburg e.V.](https://hamburg.ccc.de) ist nur der Vermittler der Touren. Für das Angebot und die Durchführung der Touren sind die jeweiligen Veranstalter verantwortlich."
     dateFormat: "02.01.2006 15:04"
     description: "Hamburg Hackertours"
     footer: "[Impressum & Datenschutz](https://hamburg.ccc.de/imprint/) – [Ticket Shop](https://tickets.hamburg.ccc.de/hackertours/39c3ht/)"
@@ -28,19 +27,19 @@ params:
 languages:
   en:
     weight: 1
-    languageName: "English"
-    languageCode: "en"
+    label: "English"
+    locale: "en"
     contentDir: content/en
     pagination:
       pagerSize: 25
     params:
-      copyright: "[CCC Hansestadt Hamburg e.V.](https://hamburg.ccc.de) only acts as an agent for the tours. The respective tour operators are responsible for the offer and running of the tours."
       footer: "[Imprint & Data Protection](https://hamburg.ccc.de/imprint/) – [Ticket Shop](https://tickets.hamburg.ccc.de/hackertours/39c3ht/)"
     title: "Hamburg Hackertours"
+    copyright: "[CCC Hansestadt Hamburg e.V.](https://hamburg.ccc.de) only acts as an agent for the tours. The respective tour operators are responsible for the offer and running of the tours."
   de:
     weight: 2
-    languageName: "Deutsch"
-    languageCode: "de"
+    label: "Deutsch"
+    locale: "de"
     contentDir: content/de
     pagination:
       pagerSize: 25