Can't collapse discussion in main thread of MR

Summary

(Summarize the bug encountered concisely)

Starting a discussion in the main thread of a MR (not on a diff) and marking it as resolved doesn't present the "Toggle discussion" button.

Steps to reproduce

• Create new project
• Create MR
• Start discussion in main discussion tab

Example Project

• issue-reproduce/mr-discussion-collapse!1

What is the current bug behavior?

Discussion cannot be collapsed like a diff discussion

What is the expected correct behavior?

Discussion can be collapsed like a diff discussion

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

reproduced on GitLab.com v11.9.0-rc9

Results of GitLab application Check

reproduced on GitLab.com v11.9.0-rc9

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)