Draft: Use bgd browser-backend's REST API for Operations-related requests
This MR replaces the use of grpc-web for Operations service requests with requests to the REST API provided by bgd browser-backend
in buildgrid!623 (merged).
The new approach uses Axios to send requests, but beyond that is broadly the same, just extracted out into a reusable place.
Draft because grpc-web is still used in CorrelatedInvocations.vue.