Add anonymous access support for private repositories (backend) (#33257)

Follow #33127

This PR add backend logic and test for "anonymous access", it shares the
same logic as "everyone access", so not too much change.

By the way, split `SettingsPost` into small functions to make it easier
to make frontend-related changes in the future.

Next PR will add frontend support for "anonymous access"

services/context/repo.go

@@ -346,7 +346,7 @@ func repoAssignment(ctx *Context, repo *repo_model.Repository) {
 		return
 	}
 
-	if !ctx.Repo.Permission.HasAnyUnitAccessOrEveryoneAccess() && !canWriteAsMaintainer(ctx) {
+	if !ctx.Repo.Permission.HasAnyUnitAccessOrPublicAccess() && !canWriteAsMaintainer(ctx) {
 		if ctx.FormString("go-get") == "1" {
 			EarlyResponseForGoGetMeta(ctx)
 			return