Adjusts build scripts to build CKEditor

[valadas]

This is a follow up PR on #4096 which merged CKEditor into this repository. This PR adjusts the build to no longer fetch it externally and build it as the rest of the project.

[bdukes]

I pushed one commit to add StyleCop.Analyzers to the packages.config for the CKEditorProvider. Once the build is done this can be merged 👍🏻

[valadas]

@bdukes I thought we had a main packages.config and projects would just reference the common packages folder, am I wrong about that?

[bdukes]

Yeah, each package has its own packages.config file (except for the couple of newer projects that use <ProjectReference /> inside the csproj file). There's a shared packages folder in the root (next to the sln), but each project needs its own list of NuGet packages (in addition to the references).

[valadas]

Hmm, ok, thanks, I guess it makes sense if you only build one project. I thought for some reason there was a single packages.config to make it easier to bump dependency verisons...