- Add option to import a Parity Signer account. It is then stored in
local storage.

- Send transactions with Parity Signer accounts

- Refactor sendStore so that "token" (and chainId) is inside this.tx;
avoids having to pass `token` around every function, and it makes sense
to put it in the object since it is part of the tx info.

- Add account type (either node or signer) to accountsInfo

- Add account type to withAccount; output props are now `{account:
{address, type}}` instead of `{accountAddress}`

lint

no drop, only browse for file

lint

move jsonbackup to its own component/route

lint

lint

todo:
* add option to export json keyfile to backup
* clean up duplicated code in store

* RPC methods$() are now called after the API is set
* Fix accountsInfo$() race condition (accountsList)
* Render component decorated with withHealth immediately (fix
"Connecting to node" not rendering)