Support deletion of an environment

[ankit-ls]

Summary

We would like to delete an environment with all the apps contained in it, with the click of a button.

Proposal

• We should be able to delete an environment with a single click.
• It should be available in Global Configs only to superadmins.
• Once clicked on Delete, it should delete all the CD pipelines for that particular environment, thereby deleting all the apps in the corresponding namespace.

AB#417