Add QR code generation and verification

[sasetz]

For each document, we need to be able to issue a QR code that can be later verified

The steps to create it:

• [x] Create a new table in the DB for secrets
• [x] Add new model and controller for the table
• [x] Add controller methods to generate QR codes
• [x] Add controller methods to verify the decoded secret

[IlliaPonomarov]

If you wanna get more practice, you can create the secrete model and based on this model create a table ( you shouldn't write sql request to create a table. Hibernate can generate automaticly)

[sasetz]

Great, thanks, will get to it tomorrow then