Stop sending full defences on low levels

[pmarsh-scottlogic]

Description

Level 1 and and 2 don't have defences. So they don't need defences stored in the backend session, nor do they need to be passed to the frontend. So I've got rid of 'em

Notes

• For level 1 and 2, we store undefined in the defences part of the session rather that a big defences object.
• While I was there, I made the endpoint validation a bit more stringent

Checklist

Have you done the following?

• [ ] Linked the relevant Issue - no issue
• [ ] Added tests
• [x] Ensured the workflow steps are passing

[pmarsh-scottlogic]

Right. I've done a bunch more stuff

• removed the weird type guarding I was doing before, and made it so that levels 3 and sandbox can be configured.
• Revamped the validation across the defenceController endpoints because it was a bit naff. (this could've been a separate ticket, or at least PR. sorry)
• Altered handleResetSingleDefence and related code (backend and frontend).
  • renamed to handleResetDefenceConfigItem
  • renamed the stuff that just said configId to configItemId
  • Now takes a level parameter to be in line with the other endpoints, even if it's technically not necessary. I think consistency is more important. Don't know if that will be a popular decision.
• adds full unit testing for defenceController
• adds integration testing for defenceController, which make sure that the right levels can and can't be configured. The reason these are integration tests and not unit tests is because a level's configurability is determined by whether its defences property is initially undefined or not.

[pmarsh-scottlogic]

I've restructured the tests in integration/defenceController.test. They now use the neat test.each() syntax that I saw in your other PR, and they're structured by controller method. The tests themselves haven't seen any change, apart from configure defence and reset config item, which now make sure that an error response is returned if on levels 1, 2 or 3.