Adds a label to social sharing links

According to the WCAG (success criterias 2.4.4 and 4.1.2), the purpose
of each link should be determined from the link text alone or from the
link text together with its programmatically determined link context.

In this case, because there is no text content, we are using
`aria-label` to have an explicit purpose announced to users of
assisting technologies.

Resources:
- https://dequeuniversity.com/rules/axe/3.2/link-name
- https://www.w3.org/TR/WCAG21/#link-purpose-in-context
- https://www.w3.org/TR/WCAG20-TECHS/ARIA8.html

templates/inc/header.tpl

@@ -18,9 +18,12 @@
         </div>
     {/if}
     <div class="social">
-        <a class="twitter" rel="noopener" href="http://twitter.com/home?status={base_url|urlencode}" title="{t}Share on Twitter{/t}" target="_blank">
+        <a class="twitter" rel="noopener" href="http://twitter.com/home?status={base_url|urlencode}" title="{t}Share on Twitter{/t}" target="_blank" aria-label="{t}Share on Twitter{/t} {t}(opens a new window){/t}">
-            <div class="twittermask"></div></a>
+            <div class="twittermask"></div>
-        <a class="facebook" rel="noopener" href="https://www.facebook.com/sharer/sharer.php?u={base_url|urlencode}" title="{t}Share on Facebook{/t}" target="_blank"><div class="facebookmask"></div></a>
+        </a>
+        <a class="facebook" rel="noopener" href="https://www.facebook.com/sharer/sharer.php?u={base_url|urlencode}" title="{t}Share on Facebook{/t}" target="_blank" aria-label="{t}Share on Facebook{/t} {t}(opens a new window){/t}">
+            <div class="facebookmask"></div>
+        </a>
     </div>
 </header>
 <div class="wrapper">