Resolve "Validierung Prüfen"

Closes #231

Please excuse the unreasonable size of the merge request (one thing, where i still have much of room for improvement). Maybe the best approach for the reviewer is to start at the latest commits and review in reversed chronological order, since the changes with the most impact were commited last.

In this MR major modifications have been made to ObservationForm.vue and HeadAndFooterDataForm.vue.
Visceral Changes:

• Massively decreased complexity and size of the two mentioned components (clear data flow, less repetition, only used code)
• Added Realtime validation of user inputs in ObservationForm and HeadAndFooterDataForm
• Increased flexibility of validation, if requirements change
• Added extra input validation checks for 'isDead' and 'outOfMethod' checkboxes, which should not be checked if no observation count is given
• Consistent re-wording of user validation error messages
• HeadAndFooterDataForm has slightly better optics
• Datetime-picker shows localized month names
• Less validation and rendering bugs