We wrap the [reset] anchor tag in a button so that we can set 'disabled'
attribute on it. We change the styles to hide the [reset] button and the
pencil icon when the widget is disabled.
We also need to call `e.preventDefault()` in the event handler since now
the anchor tag behaves as a button.
We extract a general purpose widget to create dropdown lists with
search. This widget is used for default code block language, but can
be easily extended to cover notifications_stream and similar options.
The current usage is:
```js
const widget = DropdownListWidget({
setting_name: 'realm_alpha_beta',
data: [{name: 'hello', value: 'world'}, {...}, ...],
subsection: 'msg-editing',
default_text: 'Nothing is selected',
});
```
and
```handlebars
{{> dropdown_list_widget
setting_name="realm_alpha_beta"
list_placeholder=(t 'Filter the data')
reset_button_text=(t '[Unset]')
label=admin_settings_label.realm_alpha_beta }}
```
This can further be refined by shifting more variables from handlebars
to javascript in the future.