fix(notifications): Better threading + resolve html entities decoding issue

* [x] Make Travis happy
* [ ] LGTM from 3 Elgg owners (@Srokap, @ewinslow, THREE)

Fixes #6894
For PHP5.4+, ENT_HTML5 would decode a wider variety of HTML entities. That's all.

> On Jul 8, 2014, at 11:44 PM, Evan Winslow <notifications@github.com> wrote:
> 
> In engine/lib/notification.php:
> 
> > @@ -593,13 +641,13 @@ function elgg_send_email($from, $to, $subject, $body, array $params = null) {
> >  	// Sanitise subject by stripping line endings
> >  	$subject = preg_replace("/(\r\n|\r|\n)/", " ", $subject);
> >  	// this is because Elgg encodes everything and matches what is done with body
> > -	$subject = html_entity_decode($subject, ENT_COMPAT, 'UTF-8'); // Decode any html entities
> > +	$subject = html_entity_decode($subject, ENT_QUOTES, 'UTF-8'); // Decode any html entities
> Can you elaborate? What html5 table are you talking about?
> 
> —
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