Improvements to SyndicationFeeds

I have made some adjustments/improvements based on the needs in our project, as you can see from the commit messages. Some changes in the config files are Visual Studio's work, I cannot really explain all of them. But an important update is the one in Chief2moro.SyndicationFeeds.csproj, where all references are now to ../packages. Using NuGet, it defaulted to version 7.7.0 instead of 7.7.1 when getting EPiServer.CMS.UI.Core 7.8.2 - in case you wonder why.

I hope you find the changes OK