Users of the web_service library shouldn't need to care about an
external library like json.h. However, given it's exposed in our
interface, this requires that other libraries publicly link in the JSON
library. We can do better.
By using the PImpl idiom, we can hide this dependency in the cpp file
and remove the need to link that library in altogether.
Taking them by const reference isn't advisable here, because it means
the std::move calls were doing nothing and we were always copying the
std::string instances.
* Change authentication system to JWT
* Address review comments
* Get rid of global variable, fix some documentations, fix a bug when verificating
* Refactor PostJson to avoid code duplication
* Rename jwt_token, add functionality to request a new JWT when getting a 401
* Take bools by value instead of const reference
* Send request again when JWT is invalid and use forward declarations
* Omit brackets
