Unify and simplify configuration

[ferblape]

Connects to #272

What does this PR do?

• returns secrets.yml to the repository. We don't need an example any more
• simplifies a bit the env file by removing emails configurations
• removes some default variables from the application.yml
• given that we use a different application.yml per installation, use the repository application.yml to dev and test environments only
• updates the PR template

And extra ball: removes deploy configuration.

How should this be manually tested?

Check staging works as usual & check emails address.