Streamline updateGatewayBackers usage

Investigate usage of updateGatewayBackers
- ensure it is only called once, and only called lazily if the user accesses gateway functionality (button in exchange, deposit- or withdraw modal)
- ensure it is called when error in gateway appears that could be resolved with calling it again (e.g. gateway down)

Search the JavaScript/React codebase for updateGatewayBackers and its callers, especially the exchange button and deposit or withdraw modals. Trace gateway-error handling and verify that calls are lazy, occur only once when appropriate, and can be retried for recoverable gateway failures. Done means all relevant call paths follow those rules.

Written by the indexing model from the issue text.