Update the reminder plugin and add a test for it

[nrobinaubertin]

Other changes:

• Convert tabs to spaces in the dockerfile
• Fix an exception handling in environment.py
• Create a get_mattermost_ip() function for integration tests
• Regroup common test configurations into the app() fixture
• Add a fixture to create some usual test entities
• Fix localstack version to 1.4.0 (>v2 was causing errors)
• Add an "online test" after deployment in the app() fixture. This is a workaround that should be rethought later.