feat: notify devices when Following changes (Closes #1313)

frontend/index.html

@@ -221,6 +221,8 @@
             <label class="push-update-control" for="push-updates"><input id="push-updates" type="checkbox" /> Notify me about new updates</label>
             <span class="small" id="push-update-status" role="status" aria-live="polite"></span>
             <button class="secondary" id="push-test" type="button" hidden>Send test notification</button>
+            <label class="push-update-control" for="push-following"><input id="push-following" type="checkbox" /> Notify me when Following changes</label>
+            <span class="small" id="push-following-status" role="status" aria-live="polite"></span>
             <label class="push-update-control" for="push-deadlines"><input id="push-deadlines" type="checkbox" /> Notify me about deadlines</label>
             <label for="push-deadline-hour">Reminder hour <select id="push-deadline-hour" aria-label="Deadline reminder local hour"></select></label>
             <label for="push-deadline-days">Warn me <select id="push-deadline-days" aria-label="Deadline reminder horizon"><option value="0">Due today</option><option value="2" selected>Next 2 days</option><option value="7">Next 7 days</option></select></label>