Assign issue to user

[nivu]

• assign the issue to user mapping
• table fields
  • issue id
  • userid
  • repo id
  • status
  • created_at
  • updated_at

here created at and updated at can be used to find time taken to solve the issue.

[nivu]

we need to assign mentors to the issue. so that we can keep track of mentors' contributions as well. so a separate db entry is more apt.