Allow custom sort order for environments

[rm-code]

I have checked the following:

• I've searched existing issues and found nothing related to my issue.

Describe the feature you want to add

Currently environments are sorted alphabetically.

A custom sort order would be a nice addition, especially when you are working with a lot of different environments. Current workaround is using a numbered prefix.

Mockups or Images of the feature

The nicest way would be to adopt the drag-and-drop reordering we already have in place for requests. Each environment would then have to store its sequence. As far as I can tell, this wouldn't be easy to do with the current key-value setup the environments have, but maybe it could be implemented together with #292?