Added button to empty trash in one go

When people have too many files, it is currently difficult to select
them all since the browser is unresponsive when updating so many
elements.

The empty trash button will send a simple POST request to delete ALL
files in the trashbin.

Fixes #5428 

Please review @jancborchardt @karlitschek @DeepDiver1975 

@schiesbn can you sanity check the code/approach ?

To test:
( ) Deleting lots of files + dirs with the empty trash button
( ) Delete files by selecting them (regression test)
( ) Delete files / empty trash with encrypition enabled
( ) Delete files / empty trash with version enabled and where some files have versions