Client Registration: Consider different naming

When a client configuration is downloaded, its name is always turris.conf. It has several drawbacks:

1. Some people reported that some OpenVPN clients don't accept *.conf files (probably accept only *.ovpn).
2. If multiple configurations are saved, their names must be changed manually (or are changed to turris(1).conf etc. by browsers).

I suggest to consider a diffent naming. It would use client names and ovpn as extensions, e.g. notebook.ovpn for a client named notebook.

Related to #15 (closed), https://gitlab.nic.cz/turris/foris/foris-openvpn-plugin/-/merge_requests/7/diffs

Blocked by: turris/foris-controller/foris-controller-openvpn-module#15 (moved)