Deal with user error

[ligi]

this is a meta-issue for an alternative approach to #64
the reason for suggesting #64 was mainly that in a praxis dogfooding test it showed users ignored warnings that where written in bold - it looked like this:

The idea was now to use HD wallets to prevent this user error.
But there might actually be another way.
For people that did this mistake: offer an option to sign a message with their account to prove the tx is from them and correlate this way.
This should solve most of the problems.
The rest (e.g. using an exchange that rounds) can even be small enough that manual support is feasible.
Advantage is that is is less complexity and cheaper.

[ligi]

to decide between #69 and #64 we should gather some data. Assemble a list of the error cases and gather the reason for each one (most of the time this will need us to contact the users as there is no trace of it on chain)

[ligi]

One error case is this one (just seen this once so far):

https://twitter.com/keikumata/status/1151495705699082240

Perhaps an explicit warning to not use coinbase would help here? I would actually like that ..

[ligi]

we go with #64