Return `first_activity` for Sapphire accounts

[lukaw3d]

e.g.
https://nexus.oasis.io/v1/sapphire/accounts/0x62c6ceE93cf09F52eA21F571831C1D239e30d6bD
https://nexus.oasis.io/v1/sapphire/accounts/oasis1qz78ap0456g2rk7j6rmtvasc9v2kjhz2s58qgj90

On consensus
https://nexus.oasis.io/v1/consensus/accounts/oasis1qz78ap0456g2rk7j6rmtvasc9v2kjhz2s58qgj90

"first_activity": "2022-01-22T14:38:07Z",

Unblocks oasisprotocol/explorer#1800