Add private mode: require login for reading

[DivineDominion]

For an internal wiki, we wanted to not render any page contents unless the visitor is logged-in.

So I extracted the state management logic, refactored things a bit, and introduced a new $mode = 'private'; that renders a static placeholder text.

Features

• login prompt on every page
• no login prompt for ?createUser pages

English/German i18n

Does not expose if the URL exists

With the standard set of files from this repo, /notfound shows the same placeholder. So no spying on paths!

[DivineDominion]

@til-schneider What do you think about the suggestions? We've been trying this internally and it worked quite well so far