Drop support for standalone mod components - Slice 4 - Refactor _recipe mod component property

[BLoe]

See here: https://www.notion.so/pixiebrix/Drop-support-for-standalone-mod-components-f7f3b59fc3474e45955a7b7d46baf333#1e86868c7fad4c45b66620fc30150354

• Change the signature of the ModComponentBase._recipe: ModMetadata | undefined field to be modMetadata: ModMetadata
• Migrate the types in redux-persist (It's now decoupled from the api, so this is ok)
• Clean up all the call sites and collapse all code that is currently handling _recipe == null cases

Follows on from #8979 & #8980 & #9007

[twschiller]

Closing in favor of refactoring in next phase: https://pixiebrix.slack.com/archives/C02RAMRDF0R/p1724350221764719