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

* [x] Make Travis happy
* [ ] LGTM from 3 Elgg owners (@Srokap, @ewinslow, THREE)

Fixes #6894
Github uses slashes, but let's look at [RFC](https://tools.ietf.org/html/rfc5322#section-3.6.4):
```
   this syntax only permits the dot-atom-text
   form on the left-hand side of the "@" and does not have internal CFWS
   anywhere in the message identifier.
```
Now `dot-atom-text` is defined here: https://tools.ietf.org/html/rfc5322#page-13 as allowing `/` ins strings separated by dots.