* #9 Implement routes to list, get and delete execution environments.
A refactoring was required to introduce the ExecutionEnvironment interface.
* Fix MR comments, linting issues and bug that lead to e2e test failure
* Add e2e tests
* Add unit tests
Previously, the execution.Storer interface was embedded in the Runner interface.
However, this resulted in calls like runner.Add(...) to add an execution to the
store which is kind of ugly. Thus, we decided to add only the required functions to
the runner interface and make the execution.Storer a field of the implementation.
We previously didn't really had any structure in our project apart
from creating a new folder for each package in our project root.
Now that we have accumulated some packages, we use the well-known
Golang project layout in order to clearly communicate our intent
with packages. See https://github.com/golang-standards/project-layout
