Add support of using custom env variables

[AnastasiaSliusar]

References

This PR is the part of solution of the feature where users can set up custom env variables for a kernel which has not been run yet. UI is on https://github.com/AnastasiaSliusar/jupyterlab/pull/2

Code changes

• This PR includes accept_kernel_env_var flag which can be used during launching the application. If it is true then a user can see UI for configuring custom env variables when they select another not used kernel from the kernel select dialog or when they call a context menu for a certain kernel icon on Launcher
• This PR includes support of custom_env_vars variable which has user selection and it is used to setup custom env variables for a certain kernel