Sharpened German mark and unmark translation

On iOS Safari it is difficult to click the "load images" button when "als ungelesen markieren" spans the whole row.
Semantical correctness
Added a finnish translation
Some words are shortened to better fit mobile displays
Merge pull request #382 from jukper/master

Add a finnish translation
Merge pull request #381 from snipem/master

Shorten german translation for mark and unmark
Merge pull request #383 from Weishaupt/master

Fix hiding of share items with user.css