Fix project members GraphQL

A couple of bugs have been identified when requesting projectMembers in GraphQL.

1. Searching for a user does not return any results even though they are a member of the project

   • https://gitlab.com/-/graphql-explorer

     query {
       project(fullPath: "gitlab-org/gitlab") {
         projectMembers (search:"jarka") {
           edges {
             node {
               user {
                 username
                 name
               }
             }
           }
         }
       }
     }

2. Requesting the first 5 project members returns 1 user instead of 5 users

   • https://gitlab.com/-/graphql-explorer

     query {
       project(fullPath: "gitlab-org/gitlab") {
         projectMembers (first:5) {
           edges {
             node {
               user {
                 username
                 name
               }
             }
           }
         }
       }
     }

3. Omitting first does not return 100 users (100 is the default limit: https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/graphql/gitlab_schema.rb#L31)

   • https://gitlab.com/-/graphql-explorer

     query {
       project(fullPath: "gitlab-org/gitlab") {
         projectMembers {
           edges {
             node {
               user {
                 username
                 name
               }
             }
           }
         }
       }
     }