Apply the Portland U plugin's user interface

Skinning the user interface from the version of the Mahara assignment
submission plugin developed by Philip Cali and Tony Box for
Portland University: https://github.com/fellowapeman/moodle-assign_mahara

This is based on commit 324889c9c53fe94b69f4bc9a6378a84c9ef755c2 from their repo.
Fix a bug where the currently-selected view doesn't show up
Showing the preview radio buttons
Fix filter.js to work with Firefox
Adding code to change the key column to a status column
Code for migrating the Portland U data
Fix warning about get_context_instance() deprecated
Fix the code that is supposed to exclude locked pages

It needs to exclude locked pages, unless the locked page is the currently
selected page for this assignment. Needs to work for collections as well
and tell the difference between the page & collection that have the same
ID number.
A little cleanup
Add the ability to select "no page"
Changing the links to Mahara to open in a new window
Update README.md
Update README.md