+ 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

dnsforward/dnsforward_http.go

@@ -117,12 +117,10 @@ func (s *Server) handleSetConfig(w http.ResponseWriter, r *http.Request) {
 	}
 
 	if js.Exists("upstream_dns") {
-		if len(req.Upstreams) != 0 {
-			err = ValidateUpstreams(req.Upstreams)
-			if err != nil {
-				httpError(r, w, http.StatusBadRequest, "wrong upstreams specification: %s", err)
-				return
-			}
+		err = ValidateUpstreams(req.Upstreams)
+		if err != nil {
+			httpError(r, w, http.StatusBadRequest, "wrong upstreams specification: %s", err)
+			return
 		}
 	}
 
@@ -256,6 +254,14 @@ type upstreamJSON struct {
 
 // ValidateUpstreams validates each upstream and returns an error if any upstream is invalid or if there are no default upstreams specified
 func ValidateUpstreams(upstreams []string) error {
+	// No need to validate comments
+	upstreams = filterOutComments(upstreams)
+
+	// Consider this case valid because defaultDNS will be used
+	if len(upstreams) == 0 {
+		return nil
+	}
+
 	var defaultUpstreamFound bool
 	for _, u := range upstreams {
 		d, err := validateUpstream(u)
@@ -397,6 +403,10 @@ func (s *Server) handleTestUpstreamDNS(w http.ResponseWriter, r *http.Request) {
 }
 
 func checkDNS(input string, bootstrap []string) error {
+	if !isUpstream(input) {
+		return nil
+	}
+
 	// separate upstream from domains list
 	input, defaultUpstream, err := separateUpstream(input)
 	if err != nil {
@@ -404,7 +414,7 @@ func checkDNS(input string, bootstrap []string) error {
 	}
 
 	// No need to check this DNS server
-	if input == "#" || !defaultUpstream {
+	if !defaultUpstream {
 		return nil
 	}