Disallow DELETE when index does not support delete

Since change Ic72ed4f12 it is implicitly supported to send DELETE
requests to the index REST servlets for accounts and groups, however
the account and group indexes don't support deleting entries.

Add support to reject DELETE requests. Calling DELETE in an endpoint
that does not support it now results in 405 Method Not Allowed.

Add tests to confirm that we get the expected response when sending
DELETE to the account and group endpoints.

Change-Id: I22f57b43a5ad1487cb4e11417832ad325b067ddb
4 files changed