More test coverage

[lvisintini]

• [x] Check all foreign key in the data are not broken
• [x] Check "id" values do not skip a number in the integer sequence
• [x] Check all ids are unique
• [x] Check there are no broken image file paths
• [x] Check all non-unique deployment cards have a "description" value equal to null
• [x] Checks for consistency related to the attributes "unique", "reinforce_cost" and "deployment_group" for deployment cards
• [x] Checks for consistency related to the attributes "reinforce_cost" and "deployment_group" for deployment cards
• [x] Check all "Skirmish Upgrade" deployment cards include "Skirmish" and only that value for "modes" property.
• [x] Check all "Feat"-type hero class cards have a "traits" set to null
• [x] Check all "Feat"-type reward cards have a "traits" set to null

[lvisintini]

All of the tests in the description have been added to the repo