Add simple load tests

[ricardogsilva]

This PR adds some basic load tests and integrates [locust]() into the development stack. Here is a screenshot of the locust UI when running the tests locally:

This PR also adds running the load tests in CI by means of running locust in headless mode. As such, in the CI pipeline we will now be running the load tests with:

• 20 simultaneous users
• with a spawn rate of 5 users/second
• running for 60 seconds

The CI will then fail if:

• there is more than 1% of load tests that fail
• the average response time is > 1000 ms
• the 95th percentile response time is > 2000 ms

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• fixes #203