A first pass of code standardization

[gcauchon]

The codebase greatly evolved in the last months, but this project also is a playground for member of our team learning Elixir! Even with pull reviews, things we done in many different ways… Now that we are open-sourcing the project, a little " spring cleaning " is mandatory!

1. Upgrade to latest version of elixir and erlang.
2. Update the credo configuration to match the version change.
3. Replace Poison with Jason everywhere; both where used.
4. Simplify alias with inline definition of module from the same namespace.
5. Uniform way of invoking Application.get_env/3.
6. Better logic flow using with.

7. Replace function boxing (ie def fn/1 → defp fn/2) with function header with default values.

   fn(foo, bar \\ "default")
   
   fn(nil, _) do
   …
   end
   
   fn(foo, value) do
   …
   end

[mirego-builds]

🦀 Requesting reviewers for this pull request:

• @remiprev (contributor with 17 commits in the last 90 days and 17 commits overall)
• @simonprev (reviewer for the elixir stack)

[gcauchon]

As soon as this PR is merged, I have a second one coming to apply the OTP/docker setup already available in the codebase to Heroku deployment!