Move logic from IssueType to resolvers

Previous title: Follow-up from "Add userDiscussionsCount to issues and merge requests GraphQL"

The following discussion from !46311 (merged) should be addressed:

• @minac started a discussion: (+1 comment)

  thought (non-blocking): I have the feeling that these type definitions started doing more than they should but it's not really something specific to this MR.

Implementation plan

• backend Move logic from type to resolvers
  • backend Move user_notes_count from app/graphql/types/issue_type.rb into its own resolver.
  • backend Move user_discussions_count from app/graphql/types/issue_type.rb into its own resolver.
• backend Add specs for the new resolvers
• Find other places in the code that these resolvers can be used and create follow-on issues