Support HTML comments & embedded images

[bergbria]

1. Preserves HTML in replies when adding comments to work items
2. For both replies and the original description (if the description is configured to be html), ensure that any embedded tags in the original email HTML are preserved and rendered correctly in the work item
3. Numbers 1 & 2 are both gated behind a new config settings (EnableExperimentalHtmlFeatures)
4. Number 2 currently requires including attachments when first creating the work item rather than in subsequent edits.

[msftclas]

All CLA requirements met.

[bergbria]

AngleSharp seems to be the better-maintained library at this point. After implementing with csquery, I got AngleSharp mostly working but ran into some text encoding issues (e.g. fancy quotes weren't rendered correctly). My inclination is leave csquery in use barring functional issues but am open to discussion.