Make DialogObjectForm slightly wider

At least on my laptop this allows to display two datetime-fields next
to each other. Which was previously not possible.

aleksis/core/frontend/components/generic/dialogs/DialogObjectForm.vue

 <template>
   <mobile-fullscreen-dialog
     v-model="dialog"
-    max-width="500px"
+    max-width="555px"
     :close-button="false"
   >
     <template #activator="{ on, attrs }">