Prevent creation of notes with the same `ResourceId`

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.

The issue names no files or tests. Trace the note-saving entry point and the index lookup, then determine whether duplicate ResourceId values should be rejected or notes should gain a nonce; done means the chosen behavior is implemented and verified for identical text documents.

Written by the indexing model from the issue text.