Deleting a client secret

[NvanWeeghel]

Userstory:
As a developer, I want to be able to delete a client secret so that unused secrets are not stored unnecessarily.

Acceptance criteria:

• The developer must be authorized to delete a secret.
• The expiration date of the secret must be set to the current date
• There must be confirmation before the secret can be deleted
• The status of the secret must indicate "expired".

[jmezach]

@NvanWeeghel As described in #5 I've changed the title of this story to scope it specifically to client secrets.