Similarities are optional

Also added tests for ensuring the correct behavior.

Follow-up of #151.
Editor: pass similarity data to new suggestions
Docs: updated changelog with latest changes
Prevent suggesting the same string multiple times

This prevents users from submitting suggestions that have already been
done and are in pending state.

In the front-end side simply disables the suggest button whereas in the
backend side it doesn't create any new suggestions if they already exist
in the DB in pending state.

Fixes #159.