leaflet: Reimplement coordinate box on bottom left

I'd move the event setup into addTo, but JS is utter garbage and thinks
"this" in a function callback like that should be whatever, not where you
actually define it.

overviewer_core/data/web_assets/overviewer.css

@@ -164,3 +164,12 @@ div.worldcontrol select {
     color: #333;
     font-size: 1.2em;
 }
+
+.leaflet-container .coordbox {
+    box-shadow: none;
+    font-size: 11px;
+    background: rgba(255, 255, 255, 0.7);
+    margin: 0;
+    padding: 0 5px;
+    color: #333;
+}