UI improvements for Contract Version history

[chadoh]

Follow-up to #62 / stellar-registry/indexer#47 / #40, which were all part of Q3 D12:

### D12: UI: Expose full contract version history

The Registry API now exposes full version history, which notably extends into the full history of the blockchain, beyond the launch of the Registry contract itself. This information is not yet exposed in the rgstry.xyz UI. This deliverable addresses that mismatch.

Value to ecosystem: making contract upgrades easy to find and analyze aids in troubleshooting and full-blockchain comprehensibility.

Proof: Contract detail pages on rgstry.xyz/contracts display information about full contract history.

The current approach works and communicates the information available, but it might be cleaner to follow the UI pattern from the Wasms version history side. I'm picturing:

• Go to https://stellar.rgstry.xyz/contracts/tansu
• Click "Versions" in the sidebar (we don't have this yet, but it's the pattern we're using on the Wasms pages that we could copy here)
• Land on https://stellar.rgstry.xyz/contracts/tansu/versions
• For each row currently in the versions array in the API response, see a row
• For rows that do not have named/published Wasms, you would see only the wasm hash and any other info we have, like date and what sort of event this was (upgrade/initial)

This could potentially be grouped together with other UI improvements for a Q4 milestone.