fix n+1 problem for namespaces

```
  CACHE (0.0ms)  SELECT "namespaces".* FROM "namespaces" WHERE "namespaces"."id" = $1 ORDER BY "namespaces"."id" ASC LIMIT 1  [["id", 4]]
  CACHE (0.0ms)  SELECT "namespaces".* FROM "namespaces" WHERE "namespaces"."id" = $1 ORDER BY "namespaces"."id" ASC LIMIT 1  [["id", 4]]
  Namespace Load (0.3ms)  SELECT "namespaces".* FROM "namespaces" WHERE "namespaces"."id" = $1 ORDER BY "namespaces"."id" ASC LIMIT 1  [["id", 3]]
  Namespace Load (0.3ms)  SELECT "namespaces".* FROM "namespaces" WHERE "namespaces"."id" = $1 ORDER BY "namespaces"."id" ASC LIMIT 1  [["id", 5]]
  CACHE (0.0ms)  SELECT "namespaces".* FROM "namespaces" WHERE "namespaces"."id" = $1 ORDER BY "namespaces"."id" ASC LIMIT 1  [["id", 5]]
  CACHE (0.0ms)  SELECT "namespaces".* FROM "namespaces" WHERE "namespaces"."id" = $1 ORDER BY "namespaces"."id" ASC LIMIT 1  [["id", 5]]
  CACHE (0.0ms)  SELECT "namespaces".* FROM "namespaces" WHERE "namespaces"."id" = $1 ORDER BY "namespaces"."id" ASC LIMIT 1  [["id", 5]]
  CACHE (0.0ms)  SELECT "namespaces".* FROM "namespaces" WHERE "namespaces"."id" = $1 ORDER BY "namespaces"."id" ASC LIMIT 1  [["id", 5]]
  Namespace Load (0.3ms)  SELECT "namespaces".* FROM "namespaces" WHERE "namespaces"."id" = $1 ORDER BY "namespaces"."id" ASC LIMIT 1  [["id", 7]]
  CACHE (0.0ms)  SELECT "namespaces".* FROM "namespaces" WHERE "namespaces"."id" = $1 ORDER BY "namespaces"."id" ASC LIMIT 1  [["id", 7]]
  CACHE (0.0ms)  SELECT "namespaces".* FROM "namespaces" WHERE "namespaces"."id" = $1 ORDER BY "namespaces"."id" ASC LIMIT 1  [["id", 1]]
  CACHE (0.0ms)  SELECT "users".* FROM "users" WHERE "users"."id" = $1 ORDER BY "users"."id" ASC LIMIT 1  [["id", 1]]
  Rendered users/_projects.html.haml (10.6ms)
```