Report test coverage for Django templates

[bemoody]

This adds and enables the use of django-coverage-plugin (https://github.com/nedbat/django_coverage_plugin).

With this change, code coverage reports (which are included in some of the workflow logs, though they're currently hard to access and hard to use) should include information on which template files, and which parts of the file, are exercised by the test suite.

[bemoody]

For example:

Name                                                                                        Stmts   Miss  Cover   Missing
-------------------------------------------------------------------------------------------------------------------------
console/__init__.py                                                                             0      0   100%
console/admin.py                                                                                4      0   100%
console/apps.py                                                                                 3      0   100%
console/fixtures/demo-console.json                                                              2      2     0%   1-2
console/forms.py                                                                              513    158    69%   95, 186, 203, 275, 277, 279, 317-319, 352-370, 405-407, 410-415, 418-429, 449-451, 454-459, 462-473, 504-508, 511-519, 545-547, 550-554, 557-565, 595-597, 600-604, 607-615, 668-681, 684-687, 700-703, 716-722, 725-729, 752-756, 759-781, 794-799, 827, 830, 843, 876, 908, 921-925, 933
console/migrations/__init__.py                                                                  0      0   100%
console/models.py                                                                               1      0   100%
console/services.py                                                                            32     11    66%   18-26, 32-34, 40
console/tasks.py                                                                               66     32    52%   86-87, 89, 128, 130, 138-150, 158-164, 173-179
console/templates/console/access_request.html                                                  19      2    89%   24-25
console/templates/console/add_featured.html                                                    50     36    28%   25-60
console/templates/console/application_display_table.html                                      106     10    91%   9, 13, 26, 35, 76-79, 99, 104
console/templates/console/archived_submissions.html                                            43      2    95%   37-38
console/templates/console/awaiting_authors.html                                                69     69     0%   1-83
console/templates/console/base_console.html                                                    32      0   100%
console/templates/console/code_of_conduct_detail.html                                          14      0   100%
console/templates/console/code_of_conduct_list.html                                            84      6    93%   35-36, 42-43, 77-78
...
user/widgets.py                                                                                 6      0   100%
-------------------------------------------------------------------------------------------------------------------------
TOTAL                                                                                       65000  43105    34%

[tompollard]

looks good to me!