Lazy load tile and entity background images

CSS 'background-image:url' causes the browser to synchronously load images which prevents DOM from fully loading.
Replace this with lazyload.js to make
   * js load style=background-image... after DOM is loaded and
   * only load images in viewport

The end result is much faster *apparent* page loads as all DOM is loaded before images and a reduction in load for both client/server as images are only loaded if they become visible.

maloja/web/jinja/abstracts/base.jinja

@@ -97,5 +97,10 @@
 		</div>
 
 
+    <!-- Load script as late as possible so DOM renders first -->
+    <script src="/lazyload17-8-2.min.js"></script>
+    <script>
+        var lazyLoadInstance = new LazyLoad({});
+    </script>
 	</body>
 </html>

maloja/web/jinja/partials/charts_artists_tiles.jinja

@@ -27,7 +27,7 @@
 						{% set rank = entry.rank %}
 						<td>
 							<a href="{{ links.url(artist) }}">
-							<div style='background-image:url("{{ images.get_artist_image(artist) }}")'>
+							<div class="lazy" data-bg="{{ images.get_artist_image(artist) }}"'>
 								<span class='stats'>#{{ rank }}</span> <span>{{ artist }}</span>
 							</div>
 							</a>

maloja/web/jinja/partials/charts_tracks_tiles.jinja

@@ -26,7 +26,7 @@
 						{% set rank = entry.rank %}
 						<td>
 							<a href="{{ links.url(track) }}">
-							<div style='background-image:url("{{ images.get_track_image(track) }}")'>
+							<div class="lazy" data-bg="{{ images.get_track_image(track) }}")'>
 								<span class='stats'>#{{ rank }}</span> <span>{{ track.title }}</span>
 							</div>
 							</a>

maloja/web/jinja/snippets/entityrow.jinja

@@ -8,7 +8,7 @@
 	{% set img = images.get_artist_image(entity) %}
 {% endif %}
 
-<td class='icon'><div style="background-image:url('{{ img }}')"></div></td>
+<td class='icon'><div class="lazy" data-bg="{{ img }}"></div></td>
 {% if entity is mapping and 'artists' in entity %}
 	{% if settings['TRACK_SEARCH_PROVIDER'] %}
 		<td class='searchProvider'>{{ links.link_search(entity) }}</td>