Rename 'reshare' to 'boost' in templates and PO

For now it might be enough to only reword 'reshare' to 'boost' in the
frontend. The code can continue to use 'reshare'.

Includes the most recent German translation.

Fixes #74

templates/notifications/index.html.tera

@@ -52,7 +52,7 @@
                     <i class="fa fa-retweet left-icon"></i>
                     <main class="grow">
                         <h3>
-                            {{ "{{ user }} reshared your article." | _(user=notification.object.user.name | escape) }}
+                            {{ "{{ user }} boosted your article." | _(user=notification.object.user.name | escape) }}
                         </h3>
                         <p><a href="{{ notification.object.post.url }}">{{ notification.object.post.post.title }}</a></p>
                     </main>