[BUG] Small and large shields can be built more than once

[lanedirt]

Describe the bug The small and large shield defense objects can be built more than once. They should actually only be able to be built once.

To Reproduce Steps to reproduce the behavior:

1. Go to defense page.
2. Try to build a small and/or large shield.
3. After building, try to build another unit.
4. You are able to build more than one.

Expected behavior Only being able to build maximum of 1. When one is already built, the "build" button is greyed out and disabled.

Screenshots This is what it looks like now:

This is what it looks like in the official game: