fix: use unique ID for linked accounts (#3441)

- move OAuth-related fields off of api_keys into a new user_links table
- restrict users to single form of login
- process updates to user email/usernames for OIDC
- added a login_type column to users

coderd/database/db_test.go

@@ -37,6 +37,7 @@ func TestNestedInTx(t *testing.T) {
 				CreatedAt:      database.Now(),
 				UpdatedAt:      database.Now(),
 				RBACRoles:      []string{},
+				LoginType:      database.LoginTypeGithub,
 			})
 			return err
 		})