Add upgrade & degrade action in frontend

[huodahaha]

As a project owner
I need a interface to adjust a certain customer's credit level
So that I can manage my customers easily

Assumptions:

• Upgrade and degrade are two actions

Acceptance Criteria:

Given a valid user ID
When press the upgrade button, the customer's credit level will increment by 1, and vice versa,
Also, once the credit level is lower than 0, the user should be marked as invalid.