Removed all instances of Object.assign by using es6 classes, also includes some …

What does this MR do?

Today I saw a very strange error suggesting that Object.assign was undefined. I don't know how this is possible and haven't been able to debug very deep. This is an experimental fix that removes our uses of Object.assign and uses the class keyword instead.

I also removed the eslint-disable on these files as there were just a few linter changes to make.

https://gitlab.com/gitlab-org/gitlab-ce/builds/16057434

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?