Submit holiday

[alexgutjahr]

As has been described in #50, managing holidays isn't comfortable right now. But since having them is required for our workflows, we should be able to view and add them from within trackr.

Adding holidays should be possible from the same view as is used to display them (#50). At the bottom of the page there should be three input fields, required for each holiday:

Date: Takes the date on which the holiday takes place. The field is required and can contain any valid date (future or past).
Description: A description of the holiday. The field is required, content should not be longer than 40 characters.
State: A combo-box containing all the available federal states. This field is also required. There shall also be a meta-state called All. If this state is selected, the holiday is added for all federal states.

In case there is already a holiday for a submitted date, the existing holiday will be overridden (silently). After a successful submission there should be a corresponding alert displayed to the user (cf. saving working time).

Also, the current view should be refreshed afterwards, since the holiday might be part of the current selection.