Re-allow underscore on valid_url_path_ending_chars (#4999)

Limiting allowed characters in the last character of the URL is came from twitter-text, but underscore is allowed on there, and Mastodon before #4941.

app/javascript/mastodon/features/compose/util/url_regex.js

@@ -172,7 +172,7 @@ export const urlRegex = (function() {
   , 'i');
   // Valid end-of-path chracters (so /foo. does not gobble the period).
   // 1. Allow =&# for empty URL parameters and other URL-join artifacts
-  regexen.validUrlPathEndingChars = regexSupplant(/[^#{spaces_group}\(\)\?!\*';:=\,\.\$%\[\]#{pd}_~&\|@]|(?:#{validUrlBalancedParens})/i);
+  regexen.validUrlPathEndingChars = regexSupplant(/[^#{spaces_group}\(\)\?!\*';:=\,\.\$%\[\]#{pd}~&\|@]|(?:#{validUrlBalancedParens})/i);
   // Allow @ in a url, but only in the middle. Catch things like http://example.com/@user/
   regexen.validUrlPath = regexSupplant('(?:' +
     '(?:' +