[FIX] Channel mentions when the slug starts with underscore (#4108)

patches/commonmark+0.29.0.patch

@@ -0,0 +1,13 @@
+diff --git a/node_modules/commonmark/lib/inlines.js b/node_modules/commonmark/lib/inlines.js
+index 4179cfd..478bbd5 100644
+--- a/node_modules/commonmark/lib/inlines.js
++++ b/node_modules/commonmark/lib/inlines.js
+@@ -996,7 +996,7 @@ var parseEmail = function(block) {
+     }
+ }
+ 
+-var reHashtag = XRegExp.cache('^#(\\pL[\\pL\\d\\-_.]*[\\pL\\d])');
++var reHashtag = XRegExp.cache('^#([\\pL\\d\\-_.]*[\\pL\\d])');
+ var parseHashtag = function(block) {
+     if (this.brackets) {
+         // Don't perform autolinking while inside an explicit link