devilry_subjectadmin...addgroups.AllIgnoredHelp: Require FieldSet.
devilry_subjectadmin...addgroups: Test AllIgnoredHelp.
devilry_subjectadmin...app.django.html: Set X_DEVILRY_USE_EXTJS correctly.

Avoids overwriting Ext.Ajax.defaultHeaders, which is also set by CSRF
protection.
devilry_subjectadmin...AddGroups: Include tag-help.
devilry_theme: Added bootstrap icons.
devilry_theme: Added collapse/expand classes (#351).
devilry_extjsextras...MarkupMoreInfoBox: Use the new collapse/expand classes.

Fixes a lot of #351, however some links will have to be updated with new
more/less text (remove "...")
devilry_subjectadmin...AdminsBox: Remove ``...`` (#351).
devilry_extjsextras...ContainerWithEditTitle: Style edit as button.
devilry_subjectadmin...EditGradeEditor: Use ContainerWithEditTitle.
devilry_theme: Tuned linkstyles.
devilry_subjectadmin.tests.gradeeditor: Updated for changes to the widget.
devilry_subjectadmin: Spelling.
devilry_subjectadmin...EditPublishingTime: Use containerwithedittitle.
devilry_subjectadmin: Removed all old EditableSidebarBox imports.
devilry_extjsextras...EditableSidebarBox: Removed.
devilry_subjectadmin...MoreInfoBox: Use the collapse/expand idicators.

This fixes the show/hide parts of #351.
devilry_theme: Moved bootstrap images into the bootstrap folder.
core.assignment: Use "Submission date" in error messages (#348).

Also made the error messages use more readable date formats.
devilry_subjectadmin...addgroups: Added submission date field.

It is saved before adding students. See #349.