Changing issue weight doesn't work in firefox

Summary

The weight dropdown in issues doesn't work for me on Firefox 60.2.2esr (Linux), but does work in Chromium 69.0.3497.92 (Linux)

Steps to reproduce

• Go to https://gitlab.com/gitlab-org/gitlab-ce/issues/50106
• Click "weight"

What is the current bug behavior?

I get the spinner and sometimes a very brief flash of the weight input, but am unable to see the control or enter a new weight, never mind submit

What is the expected correct behavior?

The weight controls should behave correctly in all browsers

Relevant logs and/or screenshots

At present, on GitLab.com, the page is requesting https://assets.gitlab-static.net/node_modules/dropzone/dist/basic.css which is failing with a 401 error. It does this on both chromium and firefox. Obviously, this shouldn't happen, but I don't know if it's related or not.

Possible fixes

Marking as an 11.4 regression as I'm sure I've been able to set weight in the past.

cc @victorwu @timzallmann @smcgivern @fatihacet