Adapt intersect handler to header

by introducing rootMargin.

app-bar & crud-iterator header where not included in top element
calculation.
165 px is again a magic number & should be replaced by direct
references.
But it is still not clear how to get the app-bar height.

aleksis/apps/alsijil/frontend/components/coursebook/Coursebook.vue

@@ -63,6 +63,7 @@
         v-intersect="{
             handler: intersectHandler(date, idx, lastIdx),
             options: {
+              rootMargin: '-165px 0px 0px 0px',
               threshold: [0, 1],
             },
           }"
@@ -289,7 +290,8 @@ export default {
         const entry = entries[0];
         if (entry.isIntersecting) {
 
-          if (entry.boundingClientRect.top <= 0) {
+          // TODO: Make 165 a var?
+          if (entry.boundingClientRect.top <= 165) {
             console.log('@', date.toISODate());
             this.setDate(date.toISODate());
           }