Backend infrastructure

[jankapunkt]

implements #64 and partially #62

This pull requests adds a complete code-infrastructure to the backend-server.

This includes:

• standardized setup of contexts and methods using factories
• decoupled declaration from instantiation
• decouple API-specific logic from general method calls
• use environment variables to configure API internals
• validate arguments using schema
• lint using standardx
• added initial test infra using mocha/chai + coverage

It does not include:

• Implemented sync of content, which should be done in saparate PR (see Load data from content server #57)
• Implement full tests, since we may have many changes in the next two weeks, so let's first get the whole infra ready and once this is all fixed we can add the tests
• running jsdoc, which should be done in a separate pr (see Document backend code #62)

How to run:

• the server has an own run.sh script which runs the server as expected
• it also has a test.sh script which makes it easier to define all those test parameters