Add EIP-55 capitalization to the payment address

[nagydani]

What was wrong?

Ordering my ticket to Devcon VI (pretix-powered), I noticed that the Ethereum address was all lower-case.
This makes copying the address more error-prone, as there is no checksum in Ethereum addresses beyond EIP-55 capitalization.

How can it be fixed?

Add EIP-55 capitalization to the payment address.

[ligi]

Thanks good point. We are currently revamping the plugin to single receiver mode - which (unfortunately) comes with the side-effect of having no option to manually pay. So this should be less of an issue. But still if we display Addresses it should be in ERC-55 - will see where it is still exposed after this big change.