Add support for CIDR in "$network"

I received a good tip on Slack that `propellerads` primarily rotate their domains within the CIDR range `139.45.192.0/18`, as seen with `https://securitytrails.com/list/ip/139.45.192.0%2f18`. So I initially opened up my *AdGuard for Windows* installation and tried to enter `139.45.192.0/18$network` into the "User Filters"... and saw that AdGuard refused that entry, claiming it was incorrectly formatted.

Thus I propose adding CIDR support in `$network` blocking rules.

The alternative would be for AdGuard Base to add entries against all 16,384 individual IP addresses, which I assume would be very impractical.

### Steps to reproduce

1. Go to Settings → Adblocker → User filter
2. Try to add `139.45.192.0/18$network` as a new entry.
3. See that the textbox receives red error edges and that AdGuard refuses to save the entry.

### Expected behavior

CIDR can be used in `$network` entries.

### Actual behavior

CIDR cannot be used in `$network` entries.

Screenshot:


### Customer ID

Can't find within the AdGuard program.

### Your environment

* Environment name and version: AdGuard for Windows 7.4.2
* Operating system and version: Windows 10 May 2020 Update, build 19041.388