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.

[UI Guide]: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/ui_guide.md

## What are the relevant issue numbers?

fixes #20186

## Screenshots

### Before

![before](/uploads/53ab065edb87b3e3c2f25d828b2fcc42/before.png)

### After

![after](/uploads/a9b4254d563a93098fa2f1cecb78f96f/after.png)


See merge request !5457