Fix sqlalchemy warning for activity_products__eager

[nmalaguti]

The following warning is given when running migrations:

SAWarning: relationship 'Item.activity_products__eager' will copy column
item.id to column activity_product.item_id, which conflicts with
relationship(s): 'ActivityProduct.blueprint' (copies item.id to
activity_product.item_id), 'Item.activity_products' (copies item.id to
activity_product.item_id). If this is not the intention, consider if
these relationships should be linked with back_populates, or if
viewonly=True should be applied to one or more if they are read-only.

I have applied viewonly to activity_products__eager since it appears
to only be used as a performance enhancement and is read only.

[Kyria]

yep this was done to reduce the number of query (and improve the performance) because it was a bit slow (due to the size of the tables). It's readonly, as the data itself is managed by the SDE, so it's fine.