Button: remove translation of page#317

• In the language dropdown show for which languages a content file with values exists, indicate for the others that a new content translation would be added
• Button to allow removing/resetting a content translation completely

Related: https://kirby.nolt.io/252

In the mean time when this will be added to core (😜), one can use https://github.com/junohamburg/kirby-language-selector