SPR-797 fix ID uniqueness

As explained on https://www.dokuwiki.org/devel:javascript#using_ids IDs
may not clash with headlines. To avoid that all IDs have to use two
subsequent underscores.

css/area_nav-metabox.less

@@ -1,4 +1,4 @@
-#meta-box {
+#spr__meta-box {
     position: relative;
     z-index: 10;
     display: inline-block;