Ability to automatically assign groups to users

**Is your feature request related to a problem? Please describe.**
Our locations are restricted by groups and our administrators need to manually assign a group to the users before being able to use the VPN. This is an extra step that delay the on-boarding of the user. Also, this may be frustrating for users registering with an external SSO since the admin will not be aware of his registration.

**Describe the solution you'd like**
A default group in the config of the Core should be able to be selected (optional). This will allow all users created (either manually or from SSO) to be able to directly use locations associated to the group. Giving the flexibility to be able to register and use the VPN without extra steps for the admin or the users.

**Describe alternatives you've considered**
None.

**Additional context**
None