Translation follow-up tasks

[krassowski]

Extracted out of #709 (comment):

• 
  Hide unchanged cells is in the HTML template, so I guess we need to override it in diff and merge around here:
  

  nbdime/packages/webapp/src/app/diff.ts

  Lines 258 to 261 in f044d23

  	let hideUnchangedChk = document.getElementById(
  	'nbdime-hide-unchanged',
  	) as HTMLInputElement;
  	hideUnchangedChk.checked = getConfigOption('hideUnchanged', true);

  
  

  nbdime/packages/webapp/src/app/compare.ts

  Lines 203 to 206 in f044d23

  	let hideUnchangedChk = document.getElementById(
  	'nbdime-hide-unchanged',
  	) as HTMLInputElement;
  	hideUnchangedChk.checked = getConfigOption('hideUnchanged', true);

  
  

  nbdime/packages/webapp/src/app/merge.ts

  Lines 446 to 449 in f044d23

  	let hideUnchangedChk = document.getElementById(
  	'nbdime-hide-unchanged',
  	) as HTMLInputElement;
  	hideUnchangedChk.checked = getConfigOption('hideUnchanged', true);

• unchanged cell(s) hidden, well this is defined in CSS and therefore not accessible