Multiple contract creation with entry-level slides

[leaffan]

In case of entry-level slides, i.e. the postponement of the start of a player's first contracts due to them not playing at NHL level, it can happen that the same contracts are created twice or even more often. This is because the end of a contract is included into the search parameters when trying to identify a corresponding object in the database. Due to the slide moving the end of a contract a year into the future, this kind of search is not able to find a contract in the database and therefore a new one is created. This needs to be fixed.