Responses to Adam's comments; reset common.py, i18n compliance, deleted extraneous email.html file, fixed an HttpResponse, deleted unnecessary commented-out code, some small style tweaks

set max enrollment for downloads to 200

Fix AN-115

Note: Alerts are pop-up boxes. Not sure if this is desireable, but I couldn't figure out
how to make a success message show up in the same place that error messages do.

This is in addition to email addresses, which also work.

This is a recurrent ops problem, so we wanted to make it available
on the instructor dashboard.

fix templates

A one-click way of getting to editing the course

    Added some text to guide the student to register with a particular email address.
    This is the one that was entered into the enrollment/invitation form by the instructor.
    Modified the template and associated test.