Implement BlogChain economy

[adambudziak]

This is an aggregate issue that will link to specific subissues with implementations of the economy on various levels (contracts/backend/frontend).

The description of the economy is here: #9

Tasks:

• [x] Reimplement contracts and align frontend/backend (related issue: #26)
• [ ] Show only verified stuff in the frontend (kinda done in #29)
• [ ] Improve integration of user's account with blockchain (e.g. show a total balance of a user due to actions in BlogChain)
• [ ] Add popups on transactions which will show the price of an action and ask for confirmation.