Use `join` param to load memo with payments

[bartekn]

Horizon 0.19.0 added join parameter that merges transaction data (including memo) into /payments response. This will improve loading time of memos in the payments history table.

[vcarl]

Looks like the relevant code is in history-widget-controller. This might be as simple as adding .join('transactions') to that chain there. See stellar-sdk@3.0.1 release notes for info about .join. I see we're using 2.0.1 right now, so we'll need to update as well.