Advanced UI features

[mds1]

Closes #124, closes #106 (moves one task from #106 into #135 since I want to think of the cleanest way to do that and there's more pressing functionality first). Changes:

1. Export Private Key Of Receiving Stealth Address
2. Define start and end blocks for receive Announcement scanning
3. Send to a general ETH address using recovered TX pub key, transaction hash, or specific Public Key
4. Include a Payload Extension when sending
5. Display Payload Extension on receives (should this be there even in non-advanced mode?)

Notes:

• The advanced toggle is in the footer
• Items 1, 2, and 3 are advanced-mode only feature. Items 4 and 5 are always present
• When advanced mode is on, the 🧙 emoji is shown in the top right with tooltip on hover.
• Settings such as dark mode, start and end blocks, and advanced mode toggle are stored in local storage in a new Settings store (custom private key for scanning is intentionally not stored)

[apbendi]

Overall this looks and works great! Really nice work on this— the features are all helpful. I like the memo functionality a lot and think it's good to leave it in by default.

A couple of very minor nits:

1. When exporting a private key from MetaMask, it defaults to not having the 0x prefix. Given this, I wonder if it would be better UX not to require it:

2. When typing in a memo, it's unclear how much more I will be able to type, or how much I have to delete, to fit in the space available. Due to code points in Unicode, their won't be any easy way to give a "character" limit, but I wonder if we could have some kind of progress bar or little circle-pie-chart type thing that fills up or shrinks based on the bytes used as the user types?

[mds1]

Added support for both 1 and 2 in the last two commits

[apbendi]

Looks and works great. Nice 👍