[META] phase 3

[oliverklee]

• [FEATURE] Add PHP 7.2 compatibility #159 support for PHP 7.2
• [TASK] Provide the code language in the code snippets in the README #239 provide the source code language in the code snippets in the README
• Update the composer stuff in Travis #124, [TASK] Test with both latest and oldest dependencies #241 make sure everything is fine both with the lowest as well as highest version of the dependencies
• [FEATURE] Subscription model and repository #252 add a model and repository for subscriptions
• research whether a m:n association can have an automatic creation date
• [TASK] Move test support traits to Classes/ #246, [TASK] Trait for database test, base class for web tests #264 use a shared integration test for the core and the REST API
• [CLEANUP] Always use touchDatabaseTable in the integration tests #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
• block authentication for disabled admins
• add badges for code coverage
• research how to nicely display the code quality, e.g., with badges?
• [FEATURE] System tests for testing and dev #247, #??? add system tests for all three environments
• document how to run the dev server for all 4 sub projects
• Rework the documentation #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