Refactor createTransaction/processTransaction

[katspaugh]

These two functions are problematic for many reasons.

• They share a lot of duplicate code with slight differences
• The functions are very long 🍝
• Logic is intertwined with UI notifications
• A mixture of promises, web3 events and async-await
• Lots of external parameters that could be better served by calling separate functions (e.g. execution vs signing)

Solution

• Break down the two big functions into smaller functions
• Decouple separate pieces of logic
• Extract side-effects like notifications
• Unify duplicated code across the two functions
• Create a more sensible API for components, separate the calls into:
  • tx creation w/o execution
  • off-chain signing
  • execution/on-chain signing