Add support for the DEV environment

[leplatrem]

We now have a proper DEV instance at https://settings.dev.mozaws.net/v1

We could add an entry in the environment drop-down.
The signature config (root hash) is the same as STAGE.

Loading dumps must be disabled with Services.prefs.setBoolPref("services.settings.load_dump", false);. Note that this is probably going to change in the future with the implementation of https://bugzilla.mozilla.org/show_bug.cgi?id=1702759

[matt-boris]

1.6.0 Release has been created.

Waiting on Ship-It access in order to release the new version.