Upgrade to Slim 4

[ronanchilvers]

The slim framework team are now well into the 4.x release cycle. Version 3.x is supported but will presumably be deprecated at some point. We should do the work to upgrade to Slim4.

Things to consider

• Which container? The slim bridge for ronanchilvers/container will need updating to support Slim4
• Do any controllers need reworking? We're already using PSR-7 HTTP so maybe not?
• How do we do service providers if we switch container?

[ronanchilvers]

Can now use the latest changes from ronanchilvers/slim-base as a pattern.