WIP: Implement Serilog in DNN

[donker]

This PR replaces our Log4net underpinning with Serilog. The advantages are:

1. Real-time changes in logging when editing the config file without a need for an app restart
2. Structured logging
3. The ability to add your own "sinks" (places the log is sent to)

WIP

Todo:

• Add new dlls to distribution
• Add writing out default config file to the root folder
• Add logic to repair a damaged/old config file
• Repairing all deprecated code in DNN (big effort)

[bdukes]

Thanks for all of the work on this one!