Open the UR Recruitment Google Form in a new browser tab

Based on this feedback from Joseph [1]:

> When you click ‘Sign up’, it would be better if this opened a new tab
> rather than replacing the current tab - this could be quite frustrating
> for some users who’d then need to navigate back to Signon and might
> actually cause people to close the form immediately.

[1]:  https://trello.com/c/yGMU3sR2/211#comment-64d0dc81295a6fd92483e962

app/views/root/index.html.erb

@@ -9,7 +9,7 @@
         <p class="user-research-recruitment-banner__intro govuk-body">We're holding research sessions to make Publishing work better.</p>
         <%= form_tag user_research_recruitment_update_path, method: :put do %>
           <div class="user-research-recruitment-banner__buttons govuk-button-group">
-            <button class="govuk-!-font-size-24 govuk-!-font-weight-bold govuk-button govuk-button--start govuk-button--inverse" type="submit" name="choice" value="participate">
+            <button class="govuk-!-font-size-24 govuk-!-font-weight-bold govuk-button govuk-button--start govuk-button--inverse" type="submit" name="choice" value="participate" formtarget="_blank">
               <span>Find out more</span>
               <svg class="govuk-button__start-icon govuk-!-display-none-print" xmlns="http://www.w3.org/2000/svg" width="17.5" height="19" viewBox="0 0 33 40" focusable="false" aria-hidden="true">
                 <path fill="currentColor" d="M0 0h13l20 20-20 20H0l20-20z"></path>