[55] Filter on lessons title

[danielferecatu]

What does it fix?

Issue https://github.com/code4romania/teacher-workout-backend/issues/55

How has it been tested?

From GraphiQL with seed data:

query Themes {
    lessons(term: "ip") {
        items {
            title
        }
    }
}

{
  "data": {
    "lessons": {
      "items": [
        {
          "title": "Consectetur adipiscing elit"
        },
        {
          "title": "Nemo enim ipsam voluptatem"
        }
      ]
    }
  }
}