Add the approve mentor and create category endpoints

[anjula-sack]

Purpose

The purpose of this PR is to fix #27

Goals

• Give more admin functionalities

Approach

• Add the approve reject mentor admin endpoint
• Add the create category admin endpoint

Checklist

• [x] This PR doesn't commit any keys, passwords, tokens, usernames, or other secrets.
• [x] I have read and understood the development best practices guidelines ( http://bit.ly/sef-best-practices )
• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation