Issue #292 and related to issue #195 . Considered update and create events as different cases. If updated, check all visit_numbers. Added tests accordingly

login page redirects to index if the user is already authenticated. Done in the template code since there is no view that handles this template

Issue #278 Avoid deleting availabilities from the daily planning and add more control for undefined values

Issue #195 added tests and changed the post_save function for visit. I didn't saw a problem of concurrency but a problem of saving the visit accordingly. Maybe we need more tests

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ERROR: web.tests.test_holidays (unittest.loader.ModuleImportFailure)
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ImportError: Failed to import test module: web.tests.test_holidays
Traceback (most recent call last):
  File "/usr/local/Cellar/python@2/2.7.15_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/unittest/loader.py", line 254, in _find_tests
    module = self._get_module_from_name(name)
  File "/usr/local/Cellar/python@2/2.7.15_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/unittest/loader.py", line 232, in _get_module_from_name
    __import__(name)
  File "/Users/carlos.vega/dev/scheduling-system/smash/web/tests/test_holidays.py", line 3, in <module>
    from web.management.commands.holidays import get_easter_sunday_date, get_ascension_day, get_pentecost_day
  File "/Users/carlos.vega/dev/scheduling-system/smash/web/management/commands/holidays.py", line 7, in <module>
    appointment_type_other = AppointmentType.objects.filter(code='OTHER').first()
  File "/Users/carlos.vega/dev/scheduling-system/env/lib/python2.7/site-packages/django/db/models/query.py", line 564, in first
    objects = list((self if self.ordered else self.order_by('pk'))[:1])
  File "/Users/carlos.vega/dev/scheduling-system/env/lib/python2.7/site-packages/django/db/models/query.py", line 250, in __iter__
    self._fetch_all()
  File "/Users/carlos.vega/dev/scheduling-system/env/lib/python2.7/site-packages/django/db/models/query.py", line 1118, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
  File "/Users/carlos.vega/dev/scheduling-system/env/lib/python2.7/site-packages/django/db/models/query.py", line 53, in __iter__
    results = compiler.execute_sql(chunked_fetch=self.chunked_fetch)
  File "/Users/carlos.vega/dev/scheduling-system/env/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 894, in execute_sql
    raise original_exception
ProgrammingError: relation "web_appointmenttype" does not exist
LINE 1: ...ed", "web_appointmenttype"."required_worker" FROM "web_appoi...

added option to provide a group and gather codenames together, useful to retrieve permissions from certain type such as those from configure stuff