+ upstream: Allow entering comments to the Upstreams box

Close #2083

Squashed commit of the following:

commit 113ad3c4ae2ca184b3945dcaa357b57303ee5fd6
Merge: 4ca1f005 bf23aa4d
Author: ArtemBaskal <a.baskal@adguard.com>
Date:   Tue Sep 22 14:41:30 2020 +0300

    Merge branch 'master' into feature/2083

commit 4ca1f0056708eb23bb751587a0ec284508f35edf
Author: ArtemBaskal <a.baskal@adguard.com>
Date:   Tue Sep 22 13:14:31 2020 +0300

    Simplify filterOutComments, use assert in tests

commit bba03568aa979300e0534a2bd2f03086e25b3f87
Author: ArtemBaskal <a.baskal@adguard.com>
Date:   Tue Sep 22 12:05:00 2020 +0300

    Add ValidateUpstreams test cases

commit 181de508cf266e3a47058f2b7e1b4b4accbab827
Author: ArtemBaskal <a.baskal@adguard.com>
Date:   Tue Sep 22 11:47:03 2020 +0300

    Refactor testUpstream

commit 19c189cf7b64f4d252428dec5a608595c8e4cbc7
Author: ArtemBaskal <a.baskal@adguard.com>
Date:   Tue Sep 22 10:38:37 2020 +0300

    Move functions to utils

commit 003937e90e0ff02e696d45c21045a27a49cd0202
Author: ArtemBaskal <a.baskal@adguard.com>
Date:   Mon Sep 21 19:00:49 2020 +0300

    Review changes

commit b26bf64d8cef0266f33bce51c5bad324c74bb6da
Author: ArtemBaskal <a.baskal@adguard.com>
Date:   Mon Sep 21 17:58:01 2020 +0300

    + upstream: Filter out the upstream comments

commit 920975d2ba14fade07282cdb5c72a699c8083463
Author: ArtemBaskal <a.baskal@adguard.com>
Date:   Mon Sep 21 17:51:00 2020 +0300

    Trim upstreams, extract comment token

commit a9958eb305ed9af10de68ef3bffe63f216805efe
Author: ArtemBaskal <a.baskal@adguard.com>
Date:   Mon Sep 21 10:34:11 2020 +0300

    Fix markup styles

commit 6efa41d944c7b09454a4011d2c9ea52b5ce91bbf
Author: ArtemBaskal <a.baskal@adguard.com>
Date:   Fri Sep 18 17:59:57 2020 +0300

    Fix upstream form positioning

commit 6eb12158d0bca49d4b41eb65a3ebed44eafbe486
Author: ArtemBaskal <a.baskal@adguard.com>
Date:   Fri Sep 18 17:16:49 2020 +0300

    Update example_upstream_comment locale

commit aa9317b0243f5d30f0fcb9cbfcdf502547a8e954
Author: ArtemBaskal <a.baskal@adguard.com>
Date:   Fri Sep 18 13:28:30 2020 +0300

    Highlight comments in custom rules form, extract highlight logic

commit dc55245d3db9edbde60fda0a0e50e1e045e71403
Author: ArtemBaskal <a.baskal@adguard.com>
Date:   Thu Sep 17 22:48:29 2020 +0300

    + client: Allow entering comments to the Upstreams box

client/src/components/Settings/Dns/Upstream/Examples.js

@@ -1,25 +1,10 @@
 import React from 'react';
 import PropTypes from 'prop-types';
 import { Trans, withTranslation } from 'react-i18next';
+import { COMMENT_LINE_DEFAULT_TOKEN } from '../../../../helpers/constants';
 
 const Examples = (props) => (
     <div className="list leading-loose">
-        <p>
-            <Trans
-                components={[
-                    <a
-                        href="https://kb.adguard.com/general/dns-providers"
-                        target="_blank"
-                        rel="noopener noreferrer"
-                        key="0"
-                    >
-                        DNS providers
-                    </a>,
-                ]}
-            >
-                dns_providers
-            </Trans>
-        </p>
         <Trans>examples_title</Trans>:
         <ol className="leading-loose">
             <li>
@@ -141,6 +126,12 @@ const Examples = (props) => (
                     </Trans>
                 </span>
             </li>
+            <li>
+                <code>{COMMENT_LINE_DEFAULT_TOKEN} comment</code> –&nbsp;
+                <span>
+                    <Trans>example_upstream_comment</Trans>
+                </span>
+            </li>
         </ol>
     </div>
 );