Make edit page links to open in a new tab

In a few places we opened the "edit this page" links in a new tab, in some we didn't. It's frustrating to loose the page you're on, so let's make it a default to open all "edit this page" links in a new tab.

Before:

layouts/feedback.html
15:            <a class="help-and-feedback-link" href="<%= edit_on_gitlab(@item) %>" target="_blank">Edit this page</a> to fix an error or add an improvement in a merge request<br>

After:

layouts/footer.html
6:        <a href="<%= edit_on_gitlab(@item) %>" target="_blank"><span class="text-decoration-underline">Edit this page</span></a>
7:        <a href="<%= edit_on_gitlab(@item, editor: :webide) %>" target="_blank">(<span class="text-decoration-underline">Web IDE</span>)</a>

layouts/feedback.html
15:            <a class="help-and-feedback-link" href="<%= edit_on_gitlab(@item) %>" target="_blank">Edit this page</a> to fix an error or add an improvement in a merge request<br>

layouts/archives.html
19:        <a class="btn btn-tanuki btn-default" href="<%= edit_on_gitlab(@item) %>" target="_blank">Edit this page</a>
20:        <a class="btn btn-tanuki btn-default" href="<%= edit_on_gitlab(@item, editor: :webide) %>" target="_blank">Web IDE</a>