Follower changes - allow teachers to remove students from sections

There are two options to remove a student from a section:
(1) a remove link next to the section name for each student in a section
(2) a new section in the dropdown combo box called "None" so you can multi-select students out of a section

Note that like all of the follower UI, the display strings are hard-coded in English.  I'm happy to revisit this, but I'd rather make half of the page localizable.