Add class photos_holder which is centers photos in narrow mode.

In narrow mode, it's very possible to wind up with only 1 photo per row,
and it looks silly when they are left-justified. Centered looks a little
more natural to me in this case.

frontends/etiquette_flask/static/css/common.css

@@ -12,6 +12,12 @@
     {
         --narrow: 1;
     }
+    .photos_holder
+    {
+        display: flex;
+        justify-content: center;
+        flex-wrap: wrap;
+    }
 }
 
 .hidden

frontends/etiquette_flask/templates/album.html

@@ -253,7 +253,7 @@ const ALBUM_ID = undefined;
 
         {% set photos = album.get_photos() %}
         {% if photos %}
-        <div id="hierarchy_photos" class="panel">
+        <div id="hierarchy_photos" class="photos_holder panel">
             <h3>{{photos|length}} Photos</h3>
             <div id="photo_list">
             {% for photo in photos|sort(attribute='basename', case_sensitive=False) %}

frontends/etiquette_flask/templates/search.html

@@ -371,7 +371,7 @@
 
         {{prev_next_buttons()}}
 
-        <div id="search_results_holder">
+        <div id="search_results_holder" class="photos_holder">
             {% for result in results %}
                 {% if result.__class__.__name__ == 'Photo' %}
                 {{photo_card.create_photo_card(result, view=search_kwargs["view"])}}