Also, makes small updates to `next_plan_forms_support.html`.
Removes unneeded "btn" and "btn-default" classes, and updates
the placeholder text for the input as not marked for translation.
This bug made it impossible to scrub realms where the string_id was a
number, e.g. 123.zulipchat.com because
confirmed_string_id === actual_string_id comparison failed due to one
being a string and the other a number.
Per http://api.jquery.com/data/#data-html5:
Every attempt is made to convert the attribute's string value to a
JavaScript value (this includes booleans, numbers, objects, arrays, and
null).
To retrieve a data-* attribute value as an unconverted string, use the
attr() method.
This lets us simplify the long-ish ‘../../static/js’ paths, and will
remove the need for the ‘zrequire’ wrapper.
Signed-off-by: Anders Kaseorg <anders@zulip.com>