+ client: Display upstreams list loaded from a file

Squashed commit of the following:

commit b5abc9115806f345c8d0dd5ea00b8552d27813f9
Merge: 92708411 c1c8abdd
Author: ArtemBaskal <a.baskal@adguard.com>
Date:   Mon Sep 7 14:38:06 2020 +0300

    Merge branch '1680-upstreams-file' into feature/1680

commit 92708411f07638c037e049ee8012d15bdeda77e4
Author: ArtemBaskal <a.baskal@adguard.com>
Date:   Fri Sep 4 13:54:24 2020 +0300

    + client: Display upstreams list loaded from a file

client/src/reducers/dnsConfig.js

@@ -48,6 +48,7 @@ const dnsConfig = handleActions(
         edns_cs_enabled: false,
         disable_ipv6: false,
         dnssec_enabled: false,
+        upstream_dns_file: '',
     },
 );