Refactor [breaking]: Replace currency/token 'symbol' with 'ticker'

[bvotteler]

The nomenclature around most of the stack is to use the term ticker for short-hand currency and token identifiers, rather than symbol as squid does.

Let's replace symbol with ticker for consistency, but beware this is a breaking change. So check in with the UI team, as well as Dom for Grafana dashboards which probably query for symbol, too.