HTTPotion became soft-deprecated

HTTPotion, the HTTP client we use in our integration test suite, became soft-deprecated. Should we look for an exit strategy?

## Summary

As of today, doing a `mix deps.get` results in a correct behavior but a warning comes out telling us that HTTPotion is soft-deprecated.

## Desired Behaviour

Maybe we don't want that warning, and of course we still want our libraries to be maintained.

## Possible Solution
We could use another library as the HTTP client of choice.

Possible choices that I overlooked:
- [HTTPoison](https://github.com/edgurgel/httpoison) (the basic API looks very similar to HTTPotion's)
- [Tesla](https://github.com/teamon/tesla)

Start by running `mix deps.get` and locating the integration test suite's HTTPotion dependency. Compare HTTPoison and Tesla against the current test usage, then confirm that dependency fetching no longer emits the deprecation warning while the integration tests still pass.

Written by the indexing model from the issue text.