When all planet fields are used no buildings should be able to be built anymore

[lanedirt]

PR #390 added logic to calculate the amount of used fields on a planet.

When all fields on a planet are used up the building or upgrading of buildings should be disabled. I am not sure yet what it looks like in the official game when a planet is actually full. But we could start by disallowing the upgrade via a server-side exception and improve it with the proper client-side state/warnings later.

Help wanted:
If someone knows what the official game shows when a planet is full feel free to comment.

Todo:

• Add check if all planet fields are used. If so, disallow the building or upgrading of buildings.
• This also applies to moons when Add moons #423 is merged. Moons show a warning message when the last field on a planet/moon will be used up by building the next object. Planets will probably need to show the same message.

Further notes:

• When a planet is full, the only way to upgrade a building is by demolishing/downgrading an existing building. This logic is not built yet, see issue Implement building downgrade/tear down functionality #110.
• The way to continue expanding a planet that is (almost) full is by upgrading the Terraformer building which gives additional bonus planet fields per level. This terraformer logic however is not implemented yet. A separate issue is created for this, see Implement terraformer planet field bonus logic #392.