Split rulesets into Core and Expansion

[yigitlevent]

Core

• It is standalone, meaning that it can be used without any expansions.
• At least one of these must be selected.
• It has a list of expansion rulesets that it allows.

Expansion

• Rulesets that require a Core ruleset to be enabled.
• Not standalone.
• Completely optional.

Orthagonal to this categorization, there should also be an "officiality" flag, which allows the users to see if the ruleset is official or community-made.

• Dark Sun ruleset would be [Core] [Community]
• Codex is [Expansion] [Official]

and so on.