Feature: Update pending transactions in mempool

[jxom]

Depends on #46

The Request

The ability to modify pending transactions in the mempool, with a similar UI to that introduced in #46

Problem

When a user submits a transaction, it could end up in the mempool (if mining is in "on demand" mode) – these pending transactions are seen on the "Transactions" tab with a clock icon beside them. There is currently no way to modify a transaction which is yet to be mined on a block.

Solution

As a user, I would like the ability to modify pending transactions, with a similar UI to that introduced in #46.