Fix #1635 - Escape underscores in packages for markdown

Should fix this issue https://github.com/NuGet/NuGetGallery/issues/1635

I tested it with this package https://www.nuget.org/packages/JQueryFileUpload_Demo_with_Backload/

Now he ReportPackageRequest creates such an MailBody with correct escapted underscore. Poor mans solution, but should work.

**Email:** admin (...)

**Package:** JQueryFileUpload\_Demo\_with\_Backload
http://nuget.localtest.me/packages/JQueryFileUpload\_Demo\_with\_Backload/

**Version:** 1.9.2.4
http://nuget.localtest.me/packages/JQueryFileUpload\_Demo\_with\_Backload/1.9.2.4

**User:** admin (...)
http://nuget.localtest.me/profiles/admin

**Reason:**
The package was published as the wrong version

**Message:**
test


*Message sent from NuGet Gallery*

___________
Only problem: The escaped PackageID is now also displayed in the mail subject - but I guess this is a very rare case and with this PR the link and the ID is correct in the mail body.