Port to .net core

[Pireax]

It still needs work but I've put this here for discussion.
Since .NET Framework will not receive new versions and .NET Core is the future, I think it would be a good idea to start the transition to .NET Core. A few issues (#801, #700) were already submitted about this and a few years ago a pull request (#764) was created with the same goal as this one.
Personally I want to keep the supported environments to just IIS for now. Most dependencies of the project have been ported to .NET Core by now but a few notable changes were / are necessary:

• Unity DI is discontinued and no .NET core version is available, this has been (minimally) ported to the built-in dependency injection service.
• EF code has been changed to EF Core, new API, e.g. many to many relations had to be manually created.
• Windows authentication has been commented out for now, windows authentication is built into Asp Core now, this still needs to be ported to this new method / changed to work as it is.
• Asp .NET Core also has a new way of configuration, for now I've used an App.config file to keep code changes to a minimum. This should probably be changed to an appsettings.json file
• The integration tests use SpecsFor.Mvc, this library does not support .NET Core so integration tests no longer work.
• In the unit testing department, a mocking framework is needed to test the parts that now use constructor injection and require instances of their dependencies. Moq and NSubstitute are the two big ones at the moment I think.
• I've included and ported the single CheckBoxListFor function from MvcCheckBoxList used in the codebase, in the future we could use the new tag helpers for example to replace it.

Since the settings format and the database providers have changed, this will probably not become a drop in replacement for the current Bonobo Git Server. Might make sense to create a new branch in the future for this if this is something you are interested in.