Merge branch 'new-issue-button-outline' into 'master'

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

CHANGELOG

@@ -28,6 +28,7 @@ v 8.11.0 (unreleased)
   - Bump gitlab_git to speedup DiffCollection iterations
   - Make branches sortable without push permission !5462 (winniehell)
   - Check for Ci::Build artifacts at database level on pipeline partial
+  - Make "New issue" button in Issue page less obtrusive !5457 (winniehell)
   - Add GitLab Workhorse version to admin dashboard (Katarzyna Kobierska Ula Budziszewska)
   - Add the `sprockets-es6` gem
   - Multiple trigger variables show in separate lines (Katarzyna Kobierska Ula Budziszewska)

app/views/help/ui.html.haml

@@ -189,7 +189,7 @@
             %li
               %a Sort by date
 
-        = link_to 'New issue', '#', class: 'btn btn-new'
+        = link_to 'New issue', '#', class: 'btn btn-new btn-inverted'
 
   .lead
     Only nav links without button and search

app/views/projects/issues/show.html.haml

@@ -38,7 +38,7 @@
               %li
                 = link_to 'Edit', edit_namespace_project_issue_path(@project.namespace, @project, @issue)
         - if can?(current_user, :create_issue, @project)
-          = link_to new_namespace_project_issue_path(@project.namespace, @project), class: 'hidden-xs hidden-sm btn btn-grouped new-issue-link btn-success', title: 'New issue', id: 'new_issue_link' do
+          = link_to new_namespace_project_issue_path(@project.namespace, @project), class: 'hidden-xs hidden-sm btn btn-grouped new-issue-link btn-new btn-inverted', title: 'New issue', id: 'new_issue_link' do
             New issue
         - if can?(current_user, :update_issue, @issue)
           = link_to 'Reopen issue', issue_path(@issue, issue: { state_event: :reopen }, status_only: true, format: 'json'), data: {no_turbolink: true}, class: "hidden-xs hidden-sm btn btn-grouped btn-reopen #{issue_button_visibility(@issue, false)}", title: 'Reopen issue'