[META] phase 3

[oliverklee]

• [x] #159 support for PHP 7.2
• [x] #239 provide the source code language in the code snippets in the README
• [x] #124, #241 make sure everything is fine both with the lowest as well as highest version of the dependencies
• [x] #252 add a model and repository for subscriptions
• [x] research whether a m:n association can have an automatic creation date
• [x] #246, #264 use a shared integration test for the core and the REST API
• [x] #261 use touchDatabaseTable in the ModificationDateOfNewModel and fix any bugs in touchDatabaseTable with this
• [ ] rework the login code so a web login for non-superuser admins will be possible
• [x] block authentication for disabled admins
• [ ] add badges for code coverage
• [ ] research how to nicely display the code quality, e.g., with badges?
• [ ] #247, #??? add system tests for all three environments
• [ ] document how to run the dev server for all 4 sub projects
• [ ] #237 rework the documentation
• [ ] add and configure Symfony caching
• [ ] add and configure Doctrine caching
• [ ] disable all caching for the development and testing contexts (including route caching