Make "New issue" button in Issue page less obtrusive

Summary

Every issue page has two green buttons—Comment and New Issue.

  • New Issue is not directly content related and less likely to be used.
  • UI Guide says

Use green or blue button for primary action. Primary button should be only one.

Steps to reproduce

Open any issue page.

Expected behavior

Only Comment button is a primary button.

Screenshot (after !5447 (merged))

before

Possible fixes

Use the same style as for the New Branch button: https://gitlab.com/gitlab-org/gitlab-ce/issues/18928#note_13213925

after

This would also make it consistent to the New Branch button.