Change: add settings for an explicit data privacy statement

[auge8472]

Tasks:

• add the settings to the install script
• add the settings row to the settings form
• add the necessary strings for the settings form to the language files
• add the code for procession of settings in the admin.inc.php
• add the necessary fields in the user data table
• add the code for accepting the data privacy statement in the registration form
• add the code for accepting the data privacy statement in the posting form
• add the code for accepting the data privacy statement (again) after a later introduction or a change
• redirect the user to the agreement form when one has not accepted the agreement and tries to post an entry

[auge8472]

All tasks done. I've tested the function of the parts in a more or less clean install. A review by @loesler is requested. There is no update function so far because we need two different functions for the branches legacy_2.4. and master anyway. I want to solve the mentioned conflict directly before merging as the last step.