Prevent creation of notes with the same `ResourceId`

[kirillt]

It is pretty easy to create a pair of text documents with the same content, the same content would result in the same ResourceId. We should prevent creation of resources with duplicated id.

There are 2 ways of solving this:

1. Calculate ResourceId of note being saved, lookup into the index for this id.
   If the id is already existing — forbid saving of the note.
2. Automatically add nonce to the note.
   This requires changing structure of a note — it can't be plain text anymore,
   we need to add nonce into separate field of it.

[kirillt]

Or we can just lock save button is the id already exists. User can change the content and unlock the button.