Fix cursor position after pasting text from the question area
Fix cursor position after pasting text from the question area

Summary:
https://app.asana.com/0/27216215224639/62483973765028

Weird things happen when you `preventDefault()` on a paste event and set the
textarea's contents. Here's a minimum test case reproducing the behavior:

http://jsbin.com/suqoda/2/edit?js,output

This changeset adds a bit of code after changing the question area's content
to move the cursor to the expected position.

Test Plan:
* `make serve`
* http://localhost:9000
* Add two lines of text to the question area

Case 1:
* Copy one of the lines
* Paste it between the two lines
* See the cursor is placed after the pasted text as expected

Case 2:
* Create a widget and cut it to the clipboard
* Paste it between the two lines
* See the cursor is placed after the pasted widget placeholder as expected

Case 3:
* Copy text from outside perseus (such as in another browser tab)
* Paste it between the two lines
* See the cursor is placed after the pasted widget placeholder as expected

Reviewers: emily, michelle, alex

Reviewed By: alex

Subscribers: alex

Differential Revision: https://phabricator.khanacademy.org/D23593

ariabuckles/perseus,ariabuckles/perseus,learningequality/perseus,ariabuckles/perseus,learningequality/perseus,ariabuckles/perseus