Add real buckets for tasks which don't have one (#446)

Add docs for error code

Add moving new tasks into the default bucket when none was provided

Add moving tasks in default bucket when deleting one

Fix tests again

Add test for removing a bucket

Fix tests

Prevent removing the last bucket

Remove the empty pseudo bucket

Add migration to create a new bucket for each list (and put all tasks in it

Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/446

pkg/models/error.go

@@ -1251,3 +1251,31 @@ func (err ErrBucketDoesNotBelongToList) HTTPError() web.HTTPError {
 		Message:  "This bucket does not belong to that list.",
 	}
 }
+
+// ErrCannotRemoveLastBucket represents an error where a kanban bucket is the last on a list and thus cannot be removed.
+type ErrCannotRemoveLastBucket struct {
+	BucketID int64
+	ListID   int64
+}
+
+// IsErrCannotRemoveLastBucket checks if an error is ErrCannotRemoveLastBucket.
+func IsErrCannotRemoveLastBucket(err error) bool {
+	_, ok := err.(ErrCannotRemoveLastBucket)
+	return ok
+}
+
+func (err ErrCannotRemoveLastBucket) Error() string {
+	return fmt.Sprintf("Cannot remove last bucket of list [BucketID: %d, ListID: %d]", err.BucketID, err.ListID)
+}
+
+// ErrCodeCannotRemoveLastBucket holds the unique world-error code of this error
+const ErrCodeCannotRemoveLastBucket = 10003
+
+// HTTPError holds the http error description
+func (err ErrCannotRemoveLastBucket) HTTPError() web.HTTPError {
+	return web.HTTPError{
+		HTTPCode: http.StatusPreconditionFailed,
+		Code:     ErrCodeCannotRemoveLastBucket,
+		Message:  "You cannot remove the last bucket on this list.",
+	}
+}