campaigns: move campaign spec schema to ./schema

Created by: LawnGnome

Originally, I intended the string constant in internal/campaigns to be a stopgap until a route was added to sourcegraph/sourcegraph to allow for downloading the schema, thereby allowing sourcegraph/sourcegraph to be the single source of truth for spec schemas.

However, after working more on prototyping schema changes, I've come to realise that I was mistaken, and that the tight coupling of the CampaignSpec type in internal/campaigns to the schema proper makes it — at best — extremely difficult to have the schema come from an external source.

Therefore, we need a more robust solution to handle the schema than copy/pasting the raw JSON into a string constant, getting a compile error because I forget there are backticks in the schema, and fixing those by hand. We already have the bits needed in schema to handle stringdata generation, so let's apply that here.