Avoid possibility to change the worker when adding holidays or extra availabilities. Changed the header

removed appointments reference from model. Filter out extra availabilities for the method is_on_leave

moved function to utils.py (future code will also need the midnight function) and added utils module

Fixes 204. Not sure if it's the best solution but it makes sense to call the web.views.workers the same way it's done in the sidebar

Issue #239 Removed get_details method and changed extend_context method to use Worker.get_by_user. Enhancement: now the superuser also shows a name in the upper right corner or its username if no full_name is available.

Fixes issue #237 and #232 (duplicate). Added file web/views/daily_planning.py with class TemplateDailyPlannerView