Stickers support

At Matrix it was asked if stickers are supported and as I am already on issue opening/commenting-spree I thought I could ask. However in my opinion this is very low priority issue considering how at least Conversations doesn't support them, to quote their issue tracker:

> There is a XEP for that :)
http://xmpp.org/extensions/xep-0231.html
>
> Stickers should be installed as an addidional icons pack.

* https://github.com/siacs/Conversations/issues/2122#issuecomment-262845413

> AFAIK the new [XEP-0385](https://xmpp.org/extensions/xep-0385.html) can be used for sticker caching :)

* https://github.com/siacs/Conversations/issues/2122#issuecomment-294679733

> I think a few other clients support it in a hacky, incompatible way by supporting [XEP-0071 XHTML-IM](https://xmpp.org/extensions/xep-0071.html) and then embedding data URIs. Usually done that way because it has to be done in a plugin which can't directly manipulate rendering.

* https://github.com/siacs/Conversations/issues/2122#issuecomment-405764948

Reading that issue, the XMPP clients that support stickers are:

* Movim
* Zom
* Pidgin

Start by reviewing the referenced XEP-0231, XEP-0385, and XEP-0071 specifications, along with the linked Conversations discussion. Then inspect how this bridge handles message content and media; done would require a decided, compatible sticker approach rather than simply identifying clients that support stickers.

Written by the indexing model from the issue text.