Feature: synchronize user preferences between kotlinconf site and app

[ilya-g]

Use case:

• as a user I browse kotlinconf.com before the conference and mark talks as favorites.
• then I install ConfApp and scan QR-code displayed on the site to establish a link between my app account and website session.
• after that all favorites are now synchronized between site and app. If I make changes in favorites and vote for talks, it is immediately reflected on the site.
• after the conference I can browse my favorites on the site again, for example to watch published talks I was not able to attend at the conference and to vote for them.