proposal: make models self-validate and remove Validator #121

Currently validation logic is done from Validator object. We can make models self-validate themselves.

Below is schema of Validator:

type Validator struct {
	db        *db.DB
	sanitizer markup.Sanitizer
	resolver  *idresolver.Resolver
	enforcer  *rbac.Enforcer
}

• we don't need db validation because in atproto, the target object might not be ingested yet.
• we do need to perform sanitizing check, but we can make singleton for these instead. (we are using just markup.NewSanitizer())
• we don't need did validation because of similar reason. syntax level check is enough.
• we do need to perform rbac check, but we are already doing it in most places without using centralized Validator object.

Therefore, I think we don't need a struct to group validation dependencies.

Let's ditch most of the dependencies needed to validate the model and just call Issue.Validate() instead.