Create a set of linters

[empijei]

As much as we are relying on the typesystem to do the heavy lifting, we should create linters for some things:

• ban imports (with an allowlist for exceptions) e.g. don't allow importing github.com/google/go-safeweb/safesql/legacyconversions
• ban function calls (with an allowlist for exceptions) e.g. don't allow calling safehttp.NewServeMuxConfig
• allowlists live in a configuration file (JSON)

Additional useful features:

• Make sure types that the dispatcher recognizes as safe are the only types passed to Write calls
• Make sure the return value of a function call that returns a Result type is never ignored
• Error returns shouldn't ever be ignored (people have to use explicit _ assignments). This could accept an allowlist of exceptions (e.g. io.Closer Close calls)