graph: Use userType property to distinguish between Member and Guest accounts

Fixes 5603

- Calling POST /graph/v1.0/users with userType not set will create a user as "Member"
- Calling POST /graph/v1.0/users with userType set as "Member" or "Guest" will create a user as "Member" or "Guest"
- Calling POST /graph/v1.0/users with userType set as anything but "Member" or "Guest" returns error
- Calling POST /graph/v1.0/education/users with userType not set will create a user as "Member"
- Calling POST /graph/v1.0/education/users with userType set as "Member" will create a user as "Member" and primary role as parameter specifies
- Calling POST /graph/v1.0/education/users with userType set as "Guest" will create a user as "Guest" and primary role as parameter specifies
- Calling POST /graph/v1.0/education/users with userType not set as anything but "Member" or "Guest" returns error
- Calling PATCH on /users or /education/users will update attribute in the same way as for POST
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f