stream: Add conditional note to can_add_subscribers_group.

Fixes #33156.
If the stream is set to on the private settings for privacy, we add a
parenthesis text `must be subscribed`.
We had to use JS to change the string since just having a conditional in
the handlebars template would not ensure that the parenthesis text
appears or disappears on changing the value.

tools/lib/capitalization.py

@@ -177,6 +177,8 @@ IGNORED_PHRASES = [
     r"hours",
     r"days",
     r"weeks",
+    # Used in "Who can subscribe others to this channel" label.
+    r"must be subscribed",
 ]
 
 # Sort regexes in descending order of their lengths. As a result, the