[META] phase 3

[oliverklee]

• [x] #74 Provide the code language in markdown files
• [x] #75 make sure everything is fine both with the lowest as well as highest version of the dependencies http://naderman.de/slippy/slides/2017-07-13-T3DD17-Composer-Best-Practices.pdf
• [x] #82 REST action: list of subscribers for a list
• [x] #89 REST action: get list info
• [x] document testing via Postman
• [x] #91 Use the database test trait and web test base class
• [x] #98 REST action: delete list
• [x] #101 REST action: destroy session
• [ ] REST action: update list
• [ ] #81, #??? add system tests for all three environments
• [ ] require a JSON content type for the request and add tests for requests without content type
• [ ] in the wiki or README, create a table with possible resource/action combinations and their status
• [ ] document how to test this package without the base-distribution package
• [ ] make the project structure more similar to the default Symfony project structure
• [ ] add nice 404 error JSON responses
• [ ] #32 add error handling for illegal request methods
• [ ] #57 improve the error messages for missing json content type headers and incomplete credentials
• [ ] block authentication for disabled admins
• [ ] create a REST reading list in the wiki
• [ ] #32 Add error handling for illegal request methods (MethodNotAllowedException)
• [ ] #48 Look into nelmio/api-doc-bundle for creating API docs
• [ ] research how to have different levels of detail for the response
• [ ] research how to best model actions via a REST API
• [ ] research jsonapi.org
• [ ] rework the used HTTP status codes