Email logs how have full header details

DovetailSoftware/agent/#842
DovetailSoftware/agent/#843
Parser updated to support detecting the log email header

* Refactored email headers into configurable settings
* Renderer should use discriminator on the EmailHeader
* Removed, hopefully, unused ToHtml()
Original Message parser now properly parses nested content
History html renderer was clearning content before original messages

#DovetailSoftware/agent/846
History parsing - complex email parsing only done for emails

Previously the history parser looked for email content within all history events. Now we only do it for log email in and out. Parsing is fast but I wanted to introduce original message detection that might be more intensive.
Orignal messages now use hard coded regular expressions

* Next step is to make these configurable