ModelChooserBlock label needs to be passed down Block init

[mojeto]

Setting explicit label on any RemoteModelChooserBlock doesn't work, because the value ends up on https://github.com/springload/wagtailmodelchoosers/blob/39e32d43295ace6d5c9802f88a58ddad84acff8b/wagtailmodelchoosers/blocks.py#L20

but self.label gets overwritten in parent Block.init

Also it's mandatory for label to be set on self.meta.label as well for Wagtail block to work properly. Otherwise label value gets overwritten in https://github.com/wagtail/wagtail/blob/7c3418f99e7b2c2da7ad19b2840d73099dadc6a7/wagtail/core/blocks/base.py#L104