Style loading message for OData table

[matthew-white]

We show a loading message while waiting for OData on the Submissions page and the Entity Data page. Previously, that message was styled on the Submissions page: https://github.com/getodk/central-frontend/blob/f458048622177aac3af50a00bb4d23fc32dd27b5/src/components/submission/list.vue#L381-L398. However, now that we have a generic OdataLoadingMessage component that we also use on the Entity Data page, it looks like those styles have been orphaned. I don't think we currently style OdataLoadingMessage. It'd be good to move the orphaned styles into OdataLoadingMessage.