feat: Consolidate Admin User APIs

[michael-genson]

What this PR does / why we need it:

(REQUIRED)

We have two sets of admin user routes which do the same thing. I removed some so that we only have one route per function. This is a breaking change since I have removed routes, however the remaining routes have the same function signature, so if anyone is relying on these endpoints it's an easy migration:

• GET /api/users -> GET /api/admin/users
• POST /api/users -> POST /api/admin/users
• GET /api/users/{item_id} -> GET /api/admin/users/{item_id}
• DELETE /api/users/{item_id} -> DELETE /api/admin/users/{item_id}

Note: PUT /api/users/{item_id} still remains because this is the endpoint for all users to update their own info. Maybe down the line we change this to /self (similar to the GET API).

I also added some helpful error messages for if an admin tries to create a new user with an existing username and/or email (see: #2866).

Which issue(s) this PR fixes:

(REQUIRED)

Fixes #2866

Testing

(fill-in or delete this section)

Tested the frontend to make sure admin management still works, as well as all basic user management (e.g. updating your own profile).