Undefined language constants TOP_REFERRER and TOP_EXITS lead to
uncaught error, apparently assumed to be available via
serendipity_plugin_entrylinks.
Create and use proper PLUGIN_EVENT_STATISTICS_TOP_REFERRER and
PLUGIN_EVENT_STATISTICS_TOP_EXITS constants.
Iconfont icons are of no value to screenreader users; in our case,
they get alternative text. By adding 'aria-hidden="true"' to the
<span> holding the iconfont icon, we avoid the screenreader trying
to announce the iconfont icon.
This commit is to ensure consistency with the core across the
additional_plugins repository. While I was at it, I also cleaned
up the ChangeLogs, trying to establish a common pattern.
