Change how widget adding works

Summary:
When you select a widget from the dropdown to add it we now insert the
widget where your cursor is located in the editor. If you insert a block
level widget then we'll insert newlines before/after (as appropriate),
otherwise the widget will just appear inline.

Test Plan:
Add widgets using the dropdown
Add block and inline widgets (e.g. grapher and numeric-input) and see
that the widget syntax adds new lines before and after block widgets
Try inserting widgets in various places to test the adds-new-lines
functionality

Reviewers: michelle

Reviewed By: michelle

Differential Revision: https://phabricator.khanacademy.org/D15230