Make "New issue" button in Issue page less obtrusive

What does this MR do?

Replace green background of "New issue" button on issue page by green border and text.

Why was this MR needed?

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.

What are the relevant issue numbers?

fixes #20186 (closed)

Screenshots

Before

After