closes: https://github.com/TryGhost/Toolbox/issues/299
- core/client doesn't really make sense any more now that we don't have just a client and server folder
- this folder contains ghost admin, so admin makes waaaay more sense
closes#10265
- trying to balance ease of contributing with git urls not always working
- optimising for ease of contributing for now, but #10265 has other options just in case
no issue
- import the now separated [Ghost-Admin](https://github.com/TryGhost/Ghost-Admin) project as a submodule in it's original location of `core/client`