Don't upload locally modified tasks in read-only task lists

[rfc2822]

See #572, but for DmfsTaskLists and Jtx Board collections.

Depends on #600

[github-actions]

This PR/issue depends on:

• Read-only collections: allow local changes? #600

[nexx512]

I have the same problem in combination with DmfsTaskLists and the Decks app on nextcloud. Decks provides read only calendar with tasks that is recognized as read only in davx5. By when I accidentally modify a task with my tasks app, davx5 tries to sync it leading to a 404 error because there is no put route for that task in next cloud.
From what I saw, the whole syncing for this calendar gets stuck. I have to completely disable the calendar, sync again and then enable it again to make the error disappear and to be able to sync all tasks again.