Allow new users to self-identify as a particular user type

This leverages the Django built-in Group model (which H2O did not previously use) to let users select from a predefined list of groups to which they may belong. Django Groups are typically used to gather up permissions, as for the admin, but they aren't restricted to that use and are a convenient ManyToMany relationship already available to the User.

This PR:

Runs a data migration to pre-populate the desired group types. The groups have no special behavior; they are just labels.
Runs a second migration to categorize all verified professors as group "Professor."
Enables the group mechanism in the changelist and list views of the admin—all groups are filterable in the list view, and can be modified on a per-user basis.

Smaller items:

Cleans up the list view a bit to remove less-useful fields—it was getting long enough to scroll horizontally on even a wide viewport. Also reordered the list filters to privilege the newer ones.
Sets the HTML input type for the email address field to type email. This would've happened automatically if User.email_address were an EmailField, but since it's not I set the widget type manually.
Corrects some typographic errors in the help text of the signup form and makes the error message less specific for non-educational domains, since we allow more than just .edu and .gov now.

Signup form

Looks like this now:

Admin list page

Added the new groups filter on the right, and customized the display of the groups in the list view to be comma-delimited vlaues.

Codecov Report

Merging #2001 (5886aff) into develop (41da74a) will decrease coverage by 0.01%. The diff coverage is 83.33%.

@@             Coverage Diff             @@
##           develop    #2001      +/-   ##
===========================================
- Coverage    76.83%   76.82%   -0.01%     
===========================================
  Files           62       62              
  Lines         6916     6927      +11     
===========================================
+ Hits          5314     5322       +8     
- Misses        1602     1605       +3

Impacted Files	Coverage Δ
web/main/views.py	`72.04% <ø> (ø)`
web/main/admin.py	`69.44% <80.00%> (+0.05%)`	:arrow_up:
web/main/forms.py	`90.19% <84.61%> (-0.67%)`	:arrow_down:

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harvard-lil / h2o