Toggle display for old contracts in the list

This MR fixes #49 (closed)

How it works

• A new sidebar widget (OptionWidget) is introduced to work as a filter, which toggle the display of old contracts.
• The method StundenzettelContract::findByUserIdAndInstId is introduced, which makes it possible to pass where clauses as well as order by to get the desired contracts list based on user id and institution id.
• According to the feature request, we now check if the end of contract is greater than the first day of the last month. (it might need to be approved and adjusted)
• If it is set to display the older record, we remove the where clause to fetch all records.

Note:

The German texts might need some adjustments.