Unsubscribe

[lassemand]

Purpose

Make it possible to unsubscribe all accounts from a device

Changes

Add an endpoint and call delete (Please bear in mind that device token is secret and should not be able to be guessed)

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.