Validations for User APIs

[navinpai]

User APIs have very limited validations in place, primarily targeted to the surface area of SlashGo extension. However, we need to add more validations around these APIs to handle invalid API payloads. Some examples include:

• Required fields to be provided when creating a new user
• Edit API should not allow user to edit non-admin user from editing any user other than themselves
• Admin should be able to edit any non-admin user in their own org.
• Edit API should not allow setting of any required field to null

Some of these are identified by TODO in the controller. Ideally, would be nice if this could be done alongside #2 to ensure we write tests for the new validations as well.