Feature/#157 end point for http testing

Feature

Summary

This MR temporarily adds the following end points for testing purposes to the API:

• /http-403 (will be available at https://ahikar-dev.uni-goettingen.de/api/http-403)
• /http-404 (will be available at https://ahikar-dev.uni-goettingen.de/api/http-404)
• /http-500 (will be available at https://ahikar-dev.uni-goettingen.de/api/http-500)
• /http-503 (will be available at https://ahikar-dev.uni-goettingen.de/api/http-503)

❗ This functionality is to be removed when the testing has been done.

Compliance to “Definition of Done”

• Unit tests passed
• Code reviewed
• Product Owner accepts the User Story

Documentation

• I updated the README (if applicable)
• I provided my functions with appropriate documentation
• I updated existing documentation

Tests

Are we able to test this new feature?

• Yes, everything can be done via unit tests.

Changelog

• I added a statement to the CHANGELOG.

Version number

• I bumped the version number in build.properties.

Closes

Closes #157 (closed)

/cc Mathias Göbel, Frank Schneider, Michelle Weidling