restoring tab with contributors graph shows JSON response

Summary

When browser tab with contributors graph is closed and restored, JSON is displayed instead of HTML.

Steps to reproduce

• open https://gitlab.com/winniehell/reproduction-area/graphs/master
• close tab
• restore tab (Ctrl+Shift+T)
• receive JSON

Expected behavior

• display HTML instead

Screenshot

Possible fixes

The same as for !4935 (merged): Append .json to the URL of the Ajax request here.