chat: preprocess text msgs w/md4c+tidy-html5

Introduces MessageParser to encapsulate text treatment for raw text messages.

The async parsing sequence is as follows:
- Markdown -> HTML (md4c)
- link coloration (tidy-html5)
- notify UI
- request link preview info from PreviewEngine for the first link
- Preview engine uses QtNetwork instead of QtWebengine
- Linkification is handled by MessageParser instead of linkify.js

QtWebengine is no longer required for message parsing.

Gitlab: #1033
Gitlab: #855
Change-Id: Ief9b91aa291caf284f08230acaf57976f80fa05b