popovers: Encode brackets in URI to avoid conflict with markdown links.

This fixes an issue where one could end up with a `(` in the markdown
syntax for a link after copy-pasting this, which doesn't work in
markdown.

Fixes #12579.

tools/test-js-with-node

@@ -41,6 +41,7 @@ enforce_fully_covered = {
     # 'static/js/composebox_typeahead.js',
     'static/js/dict.ts',
     'static/js/emoji.js',
+    'static/js/feature_flags.js',
     'static/js/fenced_code.js',
     'static/js/fetch_status.js',
     'static/js/filter.js',