Test more reserved usernames (#8263)

* Test more reserved usernames

* Add more reserved usernames, sort them

* Reserve "attachments" username (is a used route)

* It's "avatars" being reserved, not "avatar"

integrations/user_test.go

 	prepareTestEnv(t)
 
 	reservedUsernames := []string{
+		"admin",
+		"api",
+		"attachments",
+		"avatars",
+		"explore",
 		"help",
-		"user",
+		"install",
+		"issues",
+		"login",
+		"metrics",
+		"notifications",
+		"org",
+		"pulls",
+		"repo",
 		"template",
+		"user",
 	}
 
 	session := loginUser(t, "user2")

models/user.go

 
 var (
 	reservedUsernames = []string{
+		"attachments",
 		"admin",
 		"api",
 		"assets",