Add remove widget button

Summary: Added a 'remove' button on the header of the widget editor to quickly remove it from the question

Mostly an ice breaker for me as this is my first PR, so I used this little change (hopefully somewhat useful) as a way to get familiar with React and Perseus.

What has been done:
* Wrote a `widgetPlaceholder` string as to avoid repetition in the code
* Wrote a `widgetRegExp` string for the same reason (although it could be improved)
* Wrote a `_handleWidgetEditorRemove` function that removes the selected widget from the textarea content and calls re-rendering
* Placed a remove button in the header of the `WidgetEditor`
I'm not sure I understand how you would go about this, isn't it necessary to create the RegExp on the fly as we need to do a multi-line replacement and we don't know the `id` of the widget until the user clicks on the remove button?