* BTCPay Extensions Part 2
This PR cleans up the extension system a bit in that:
* It renames the test extension to a more uniform name
* Allows yo uto have system extensions, which are extensions but bundled by default with the release (and cannot be removed)
* Adds a tool to help you generate an extension package from a csproj
* Refactors the UI extension points to a view component
* Moves some more interfaces to the Abstractions csproj
* Rename to plugins
* Preparing final version of CircleCI docker building
* Removing test job requirement for building Docker images
We'll already monitor build before tagging, would be too many checks
* Adding pushing of manifest for tag
* Easy access to docker/circle config files for edit
* Generalizing script with $DOCKERHUB_REPO variable
