Fix inconsistency in youtube channel urls

Urls from the youtube search extractor were "https://www.youtube.com/user/NAME" instead of "https://www.youtube.com/channel/ID". This fixes TeamNewPipe/NewPipe#2167

Author: Stypox

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/extractors/YoutubeChannelExtractor.java

@@ -47,6 +47,7 @@
 
 @SuppressWarnings("WeakerAccess")
 public class YoutubeChannelExtractor extends ChannelExtractor {
+    /*package-private*/ static final String CHANNEL_URL_BASE = "https://www.youtube.com/channel/";
     private static final String CHANNEL_FEED_BASE = "https://www.youtube.com/feeds/videos.xml?channel_id=";
     private static final String CHANNEL_URL_PARAMETERS = "/videos?view=0&flow=list&sort=dd&live_view=10000";
 
@@ -72,7 +73,7 @@ public String getNextPageUrl() throws ExtractionException {
     @Override
     public String getUrl() throws ParsingException {
         try {
-            return "https://www.youtube.com/channel/" + getId();
+            return CHANNEL_URL_BASE + getId();
         } catch (ParsingException e) {
             return super.getUrl();
         }

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/extractors/YoutubeChannelInfoItemExtractor.java

@@ -5,6 +5,9 @@
 import org.schabi.newpipe.extractor.exceptions.ParsingException;
 import org.schabi.newpipe.extractor.utils.Utils;
 
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
 /*
  * Created by Christian Schabesberger on 12.02.17.
  *
@@ -53,8 +56,20 @@ public String getName() throws ParsingException {
 
     @Override
     public String getUrl() throws ParsingException {
-        return el.select("a[class*=\"yt-uix-tile-link\"]").first()
-                .attr("abs:href");
+        String buttonTrackingUrl = el.select("button[class*=\"yt-uix-button\"]").first()
+                .attr("abs:data-href");
+
+        Pattern channelIdPattern = Pattern.compile("(?:.*?)\\%252Fchannel\\%252F(.+?)\\%26(?:.*)");
+        Matcher match = channelIdPattern.matcher(buttonTrackingUrl);
+
+        if (match.matches()) {
+            return YoutubeChannelExtractor.CHANNEL_URL_BASE + match.group(1);
+        } else {
+            // fallback method just in case youtube changes things; it should never run and tests will fail
+            // provides an url with "/user/NAME", that is inconsistent with stream and channel extractor
+            return el.select("a[class*=\"yt-uix-tile-link\"]").first()
+                    .attr("abs:href");
+        }
     }
 
     @Override