[Feature] Default environment

JIRA

Currently, when you open a request for collection, you always have to first set the environment.

I suggest we have a default environment that will be used upon opening a request collection if user didn't set anything. It can be settable either in collection properties (selecting existing environment) or in environment config (ticking a default checkbox).

This way, executing requests can be made to work immediately upon opening them and CLI will work without mandatory --env parameter.