Buttons should use material-design-icons

[raimund-schluessler]

The buttons here

calendar-availability-vue/src/CalendarAvailability.vue

Lines 26 to 29 in a9776d4

	<button :key="`slot-${day.id}-${idx}-btn`"
	class="icon-delete delete-slot button"
	:title="l10nDeleteSlot"
	@click="removeSlot(day, idx)" />

and here

calendar-availability-vue/src/CalendarAvailability.vue

Lines 38 to 42 in a9776d4

	<button :key="`add-slot-${day.id}`"
	:disabled="loading"
	class="icon-add add-another button"
	:title="l10nAddSlot"
	@click="addSlot(day)" />

should use material-design icons to be in line with the rest of the Calendar app.

Since the new button component from nextcloud/vue@5 is not released yet, one option would be to use ActionButton instead. Similar to how it is done in the trash bin:
https://github.com/nextcloud/calendar/blob/master/src/components/AppNavigation/CalendarList/Trashbin.vue#L88-L96