Api key implementation

[AliasXoX]

This pull request implements a new authentication method for lea5 using API Keys. The authentication procedure using an API key has been documented. Here's an overview of the changes coming with this pull request :

• A new controller and view in order for admin to see, create and delete API keys
• A new rails concern to authorize API key authentication (see api_key_authenticatable.rb) (to understand : https://keygen.sh/blog/how-to-implement-api-key-authentication-in-rails-without-devise/)
• A new route to login as an api key bearer (see routes.rb ; sessions_controller.rb ; sessions_helper.rb)
• A new rails secret to hash API keys (see api_key.rb)
• An encoded file for credentials for the test environment
• Modifying the way Cancancan abilities work in lea5 (see user_ability.rb ; user_ability.rb ; application_controller.rb)
• Some tests that cover abilities, login with an api key and of course the api key controller's actions

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (01e1374) to head (3587ade).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #466 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 26 31 +5 Lines 337 409 +72 Branches 35 42 +7 ========================================= + Hits 337 409 +72 ```

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