Stop using Pimple as service locator

[jakobmats]

Pimple should not be used as a locator for fetching dependencies from as things are starting to get coupled which obviously is the exact opposite of the problem that it was supposed to solve.

• Use Symfony DI instead,
• get rid of service providers,
• find a way to autowire the dependencies in the HTTP verb methods and middlewares.

[jakobmats]

Implemented starting with 3e365f2

[jakobmats]

Things to note:

• Dependencies are now autowired in controller classes (which are plain PHP classes),
• Route callbacks have access to the container instance from which dependencies can be fetched (service locator).