Add JobLevels feature

[amaurymedeiros]

Add a level field to User model.

JobLevels model should contain the following fields:

• name (required)
• description (optional)
• skills (optional) necessary to reach the level
• responsibilities (optional) of someone at this level
• openings (optional).

JobLevels controller should include the following methods:

• create (ADMIN only)
• update (ADMIN only)
• delete (ADMIN only. ADMIN level can't be deleted)
• list
• show

Suggested tests:

• Create level using an ADMIN user (should return 201 - Created)
• Create level using a user from any other level (should return 401 - Unauthorized)
• Update level using an ADMIN user (should return 200 - OK)
• Update level using a user from any other level (should return 401 - Unauthorized)
• Delete level using an ADMIN user (should return 200 - OK)
• Delete level using a user from any other level (should return 401 - Unauthorized)
• List all levels using an ADMIN user (should return 200 - OK)
• List all levels using a user from any other level (should return 200 - OK)
• View a level using an ADMIN user (should return 200 - OK)
• View a level using a user from any other level (should return 200 - OK)
• Create/Update feedback without name field