feat: add calendar export functionality and option to create ICS file for appointments

Web/templates/termin_configure.html

@@ -51,6 +51,12 @@
                             <textarea id="note" name="note" class="form-control" rows="4" placeholder="Optionaler Einführungstext für die Mail"></textarea>
                         </div>
 
+                        <div class="form-check">
+                            <input class="form-check-input" type="checkbox" id="add_to_calendar" name="add_to_calendar" {% if add_to_calendar %}checked{% endif %}>
+                            <label class="form-check-label fw-semibold" for="add_to_calendar">Kalendereintrag für den Nutzer erzeugen</label>
+                            <div class="form-text">Erzeugt zusätzlich eine .ics-Datei, die in gängige Kalender importiert werden kann.</div>
+                        </div>
+
                         <div class="d-flex flex-column flex-sm-row gap-2 pt-2">
                             <button type="submit" class="btn btn-primary btn-lg">Terminplan erzeugen</button>
                             <a class="btn btn-outline-secondary btn-lg" href="{{ url_for('terminplaner.main') }}">Zur Übersicht</a>
@@ -70,6 +76,12 @@
                     {% endif %}
                 </div>
                 <a href="{{ generated_link }}" class="d-inline-block text-break">{{ generated_link }}</a>
+                {% if calendar_link %}
+                <div class="mt-3">
+                    <div class="fw-semibold mb-1">Kalendereintrag</div>
+                    <a href="{{ calendar_link }}" class="btn btn-outline-primary btn-sm">.ics herunterladen</a>
+                </div>
+                {% endif %}
             </div>
             {% endif %}
         </div>