create proposal (not amendment) from classic norm

This changes the way the "propose new amendment" button in variants view works. The basic problem is as follows:

-   If a norm is in sectionpage mode we want proposals
-   If a norm is in classic mode we want amendments
-   The mode of a norm may be changed later
-   The mode of a proposal (amendment or not) can not be changed later.

We have decided that in practice the mode of a norm will not be changed.

Whether or not a proposal should be an amendment to a norm therefore depends on the mode of that norm at the time of creation. This implements the needed changes.