Add validation to the authorities forms

Verified on 501-create-authorities-page branch.
We need better error messages for validation authorities form

• unknown user (unintelligible message)
  
• Too long account name (better message needed)
  
• Incorrect key (better message needed)
  
• Too short key (better message needed)
  
• Too long key (better message needed)
  
• Signing authority changes with the wrong key (better message needed)
  
• Unnecessary zero when clearing input and entering new value