Add support for rendering basic HTML in TalkatuHistory.

* Remove the data argument from class methods
* Renamed TalkatuHtmlParser to TalkatuHtmlRenderer
* Created TalkatuHtmlPangoRenderer
* Use the TalkatuMessage::content-type to determine how to render the text in
TalkatuHistory.

Testing Done:
Tested in the demo with a hack for the content type that will be fixed in a later review request.

Reviewed at https://reviews.imfreedom.org/r/403/