Add GraphQL API to Create Saved Reply

What does this MR do and why?

This MR adds ability to create and list Saved Replies for current user using GraphQL API.

How to set up and validate locally

1. Enable feature flag for your user in rails console:

Feature.enable :saved_replies, User.find(your_user_id)

2. Go to GraphQL Explorer to create saved replies:

mutation {
  savedReplyCreate(input: { name: "test_111", content: "Test 12311" }) {
    savedReply {
      content
      id
      name
    }
    errors
  }
}

3. Go to GraphQL Explorer and query saved replies:

query {
  currentUser {
    savedReplies {
      nodes {
        id
        name
        content
      }
    }
  }
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #352938 (closed)