Add 'Default poll method' for motions

[Elblinator]

Since OpenSlides/openslides-client#3979 a motion poll can be method YN and YNA

Wanted behaviour:
Like the election "Default election method" the motions need a default setting for their poll method.

Please add a new setting with a similar key to "motion_poll_default_method"

Like the assignment_poll_method this new setting should save the default method a motion poll is created with