Change markdown italic toolbar button to use `_`

instead of `*`.  This makes it more consistent
with selected text and keystroke `*`
for bolding and keystroke `_` for italics

app/assets/javascripts/vue_shared/components/markdown/header.vue

@@ -129,7 +129,7 @@ export default {
       <li :class="{ active: !previewMarkdown }" class="md-header-toolbar">
         <div class="d-inline-block">
           <toolbar-button tag="**" :button-title="__('Add bold text')" icon="bold" />
-          <toolbar-button tag="*" :button-title="__('Add italic text')" icon="italic" />
+          <toolbar-button tag="_" :button-title="__('Add italic text')" icon="italic" />
           <toolbar-button
             :prepend="true"
             :tag="tag"

app/views/shared/blob/_markdown_buttons.html.haml

 .md-header-toolbar.active
   = markdown_toolbar_button({ icon: "bold", data: { "md-tag" => "**" }, title: _("Add bold text") })
-  = markdown_toolbar_button({ icon: "italic", data: { "md-tag" => "*" }, title: _("Add italic text") })
+  = markdown_toolbar_button({ icon: "italic", data: { "md-tag" => "_" }, title: _("Add italic text") })
   = markdown_toolbar_button({ icon: "quote", data: { "md-tag" => "> ", "md-prepend" => true }, title: _("Insert a quote") })
   = markdown_toolbar_button({ icon: "code", data: { "md-tag" => "`", "md-block" => "```" }, title: _("Insert code") })
   = markdown_toolbar_button({ icon: "link", data: { "md-tag" => "[{text}](url)", "md-select" => "url" }, title: _("Add a link") })

spec/features/issues/markdown_toolbar_spec.rb

@@ -36,6 +36,6 @@ RSpec.describe 'Issue markdown toolbar', :js do
 
     all('.toolbar-btn')[1].click
 
-    expect(find('#note-body')[:value]).to eq("test\n*underline*\n")
+    expect(find('#note-body')[:value]).to eq("test\n_underline_\n")
   end
 end