Fix for XSS vulnerability

ClearIT informed us about a video from customer ComHem that showed
how a carefully constructed message which includes a as of right
now unknown Javascript file that allowed the customer from the External.app chat
see the agents username and password.

The code now escapes a few characters with HTML entity encoding to prevent this sort of attack.
The code escapes the following characters

& into &amp;
< into &lt;
> into &gt;
" into &quot;
' into &#x27;
/ into &#x2F;

This has been done according to the recommendation found here:
https://www.owasp.org/index.php/XSS_%28Cross_Site_Scripting%29_Prevention_Cheat_Sheet