More control over user devices

**Is your feature request related to a problem? Please describe.**
Users should only be able to configure one (or a limited number of) devices.
For example: Every user should have a single notebook that they use for work, with the DefGuard client active only on that device. It should not be possible to use the client on multiple devices. As a workaround, a user could uninstall the client and reinstall it on another device, but this is not a desired behavior.

**Describe the solution you'd like**
- Users should only be able to configure one (or a limited number of) devices.
-> Disable self-enrollment and send the token via email to admins upon request. (?)
- Prevent the deletion of the configured device to avoid re-configuration on another device.
- Provide an option to disable phone configuration.
- Allow exceptions for certain users to configure more devices, enable phone configuration, self-enrollment, etc.
- Admins might have to make sure that the token is enabled on the work notebook (hostname + serial number of device?)

**Describe alternatives you've considered**
None

**Additional context**
In general, each basic user should only have the DefGuard client on their work notebook. If a user gets a new notebook, the admin should assist with reconfiguration, unless another solution is implemented. Special users, such as the CEO, may require exceptions for additional devices, like a second notebook or a phone.

Depending on the progress of [Issue #955](https://github.com/DefGuard/defguard/issues/955), there may already be partial solutions for this feature.