Allow admin to delete user keys via API

[vtemian]

• Gitea version (or commit ref): 6ad4990
• Git version: 2.7.4
• Operating system: Ubuntu 16.04
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL)
  • No
  • Not relevant
• Log gist:

Description

Right now, admins can create keys on behalf of an user, but they can't delete them. (via API)
I suggest that we also support the DELETE operation.

From a bigger perspective, is there any reason why the /user endpoints where implemented instead of /users/{username}? (I guess they were already implemented in gogs)

It's easier for an admin to manage it's users via /users/{username}. It can introduce some overhead and confusion if for each resource type endpoint will exist an equivalent admin endpoint (DELETE: /users/{username}/keys/{id} -> /admin/users/{username}/keys/{id}).