Incorporate post-MVP actions

[ltfschoen]

Important note: This commit 845ff71 is incomplete.
There are some FIXMEs that need resolving:

• 845ff71#diff-fc2de32b6ca1cc4e72684954b1cba8d4R21
• 845ff71#diff-30da1c1ce14469ebff63f78290faba28R30

The API aspect has been put on hold since for MVP it was agreed on 7th April 2020 that the Flutter app for the testnet would just show instructions and only use a front-end web3 library to read and show the state from the Ethereum smart contracts that record the lock and signals (i.e. show the MXC and IOTA-E balance. Excluded from MVP is having functionality to directly lock, signal, and claim from within the app.