link-preview: fix UTF-8 decoding

The HTML string was being converted to a local 8-bit string prior to parsing with tidy.

According to Qt documentation, this works on Unix systems as toLocal8Bit() is equivalent to toUtf8(), however, it may not on Windows, and may result in character replacement.

Gitlab: #1536
Change-Id: I7aec98cffbd9d72771d1c489f52b38dbb3a1ed9f