Fix missing SRD monster entries

[ben]

Fixes #445. We've got YAML for them, but they don't have _id fields at the top level for whatever reason. Here's a list of things to fix that I found by running npm run build | grep undefined:

• [x] Elemental beast template: earth
• [x] Elemental beast template: fire
• [x] Elemental beast template: water
• [x] Barbed Vines
• [x] Big Flux Elemental
• [x] Bone Imp Archer
• [x] Bone Imp Mage-Eater
• [x] Bone Imp Skullbiter
• [x] Bone Imp Tormentor
• [x] Briar Elf Assassin
• [x] Briar Elf Slayer
• [x] Briar Elf Sorcerer
• [x] Briar Elf Sovereign
• [x] Briar Elf Stalker
• [x] Briar Elf Warrior
• [x] Briar Tree
• [x] Coin Collector Zombie
• [x] Coin Zombie
• [x] Derro Berserker
• [x] Epic Flux Elemental
• [x] Great Horned Owlbear
• [x] Greater Flux Elemental
• [x] Jackpot Zombie