Refactor Booster System Internals

[taw]

Very little of it should be visible, it's mostly internal stuff for me.

• [x] remove PatchShowcase
• [x] remove all references to is:booster from yaml files
• [x] move name metadata from indexer/lib/patches/patch_has_boosters.rb to booster_indexer/lib/booster_indexer.rb
• [x] copy any relevant comments from old code to yaml files
• [x] delete old booster code
• [x] rewrite the specs to use new code when applicable
• [x] don't use old metadata for list of boosters
• [x] make is:booster use the new system (= booster:*)
• [ ] create some kind of a cache for booster data to avoid slow server start

There's probably going to be more cleanup to do, but that's a start.

[taw]

Booster names are now handled by booster indexer, for any custom ones they can be overriden by header like name: "Alara Premium Foil Booster". All default classes are set by indexer.

Small complication is that indexer is loaded before mtgjson data, so it doesn't know set names, only their codes, so names are all like {set_name} Arena Booster and {set_name} only gets replaced by final value when loaded.

[taw]

Closing this as done enough. I might still add a cache for precomputed booster data at some point.