Add dropdown to choose a default role for MySQL 8.0 user

**Is your feature request related to a problem? Please describe.**
In MySQL 8.0.x a user can be set a default role using ALTER USER ... DEFAULT ROLE command.

**Describe the solution you'd like**
Add a dropdown box for choosing a default role from a list in the user properties screen.
The role should be granted and set as default when saving the user properties.

**Describe alternatives you've considered**
Running:
```
GRANT [role] TO [user];
ALTER USER [user] DEFAULT ROLE [role];
```