Fix frontend and other minor changes

[cstybert]

• Frontend now has a automatically-updating "Pending Transactions" list. This list contains user-submitted transactions (create graph/ execute activity).
• Backend now has PendingTransactions list. Same as above. Backend will automatically notify frontend when a transaction has been processed.
• Backend now exposes a PendingTransactions endpoint (dcr/pending) for fetching pending updates.
• Backend now saves blockchain to "blockchain.json", e.g. "blockchain4300.json".
• Since DCR actions like creating graphs and executing them are allowed only for FullNodes, DCRController now takes FullNode as argument instead of AbstractNode.