Align various controllers

[kjac]

🚨 DO NOT MERGE THIS UNTIL THE CLIENT IS READY 🚨

As this PR moves things around, it is breaking toward the client, so it needs to catch up before we can merge this PR.

Prerequisites

• I have added steps to test this contribution in the description below

Description

There are a few controllers that look or behave differently than the rest.

Do not use "Entity" in naming

The language and webhook item controllers should follow the current standard naming wise - use "Item", not "Entity.

Use the "/item" silo

The webhook item endpoint is placed incorrectly - it should be located under the "/item" silo to follow the conventions of the API.

Use singular naming

The user group controller base is named correctly (singular) in code, but the file is plural.

Follow response standards

1. The member group create endpoint should yield a 201 Created response upon a successful member group creation.
2. The member group update endpoint should yield an empty 200 OK response upon a successful member group update.

Testing this PR

Use Swagger to test all affected endpoints.

[elit0451]

Looks good 💪