Reorganise templates. Closes #246

aleksis/core/templates/403.html

aleksis/core/templates/404.html

aleksis/core/templates/500.html

aleksis/core/templates/503.html

aleksis/core/templates/core/base.html

aleksis/core/templates/core/base_print.html

aleksis/core/templates/core/group/child_groups.html

aleksis/core/templates/core/group/edit.html

aleksis/core/templates/core/group/full.html

aleksis/core/templates/core/group/list.html

aleksis/core/templates/core/grouptype/edit.html

aleksis/core/templates/core/grouptype/list.html

aleksis/core/templates/core/index.html

aleksis/core/templates/core/management/data_management.html

aleksis/core/templates/core/pages/about.html

aleksis/core/templates/core/pages/offline.html

aleksis/core/templates/core/pages/system_status.html

aleksis/core/templates/core/partials/admins_list.html

aleksis/core/templates/core/partials/announcements.html

aleksis/core/templates/core/partials/crud_events.html