Fix contest creation and make it easier

[Toastbrot236]

This PR is meant to close #1005 by fixing a few bugs in contest creation, while also making it easier by doing the following changes:

• Request attributes like organizer ID and template level ID are now properly sanitized when creating or updating a contest, with the server returning proper error messages and the latter not throwing if it points to an invalid ID.
• Error messages for contest creation and updating are hopefully more helpful now
• Missing template level IDs from creation requests are now also properly handelled.
• Titles, start and end dates are now explicitly checked to be included alongside organizer ID in creation requests, while other attributes will fall back to default values.
• If there are no allowed games specified for a contest, all games are allowed.
• The creation database method now receives an object implementing ICreateContestInfo instead of a GameContest to make it more consistent with the creation methods of other entities

[maks2011-2017]

dude whaaat, you are the goat!
@jvyden , you gotta probably check on PRs... maybe? eventually??😅