Add user types admin, moderator & mentor

### User types
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#### 1. Admin

Andela staff, mostly the Program Manager who oversees the end-to-end execution of a learning program. He coordinates all resources to ensure the program is delivered to meet the set out goal.

#### 2. Mentor Manager

Program Assistants who assist program manager in the execution of a learning program. They oversee the activities of mentors and interact directly with them. They are volunteers and they are expected to access the system via their personal email account.

#### 3. Mentor

Mentors are volunteers who interact directly with the learners with the sole responsibility of unblocking the learners and encouraging them to meet learning goals. Usually, we have a mentor to a group of 10 -20 learners.