add localisation support

currently only en_GB (TODO: dynamic language pack imports)

src/lib/ui/post/PostHeader.svelte

@@ -1,5 +1,8 @@
 <script>
     import { shorthand as short_time } from '$lib/time.js';
+    import Lang from '$lib/lang.js';
+
+    const lang = Lang('en_GB');
 
     export let post;
     export let reply = undefined;
@@ -19,10 +22,8 @@
         <div class="post-info" on:mouseup|stopPropagation>
             <a href={post.url} target="_blank" class="created-at">
                 <time title={time_string}>{short_time(post.created_at)}</time>
-                {#if post.visibility !== "public"}
-                    <br>
-                    <span class="post-visibility">{post.visibility}</span>
-                {/if}
+                <br>
+                <span class="post-visibility">{lang.string('post.visibility.' + post.visibility)}</span>
             </a>
         </div>
     </header>