Adapt front end to new RPC-based joinable tables API

[seancolsen]

Notes

• I did some light refactoring within some of the UI components while working on this. Just tidying a few things up.

• I removed the tests for getLinkFromColumn because adapting the mock data to suit the new API response structure didn't seem worth the time. I could be convinced to fix it though if you think those tests are important enough to preserve.

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[pavish]

@seancolsen Looks good.

I haven't tested this on the data explorer since tables.get_with_metadata doesn't seem to be implemented yet, which the data explorer requires.

I tested it on the inspector and the record page with the Library Management dataset and noticed this error when opening the Record page.

Widgets.svelte:27 Uncaught (in promise) TypeError: Cannot destructure property 'name' of 'joinableTablesResult.target_table_info[table.oid].columns[fkColumnId]' as it is undefined.
    at buildWidgetInput (Widgets.svelte:27:13)
    at Array.map (<anonymous>)
    at $$self.$$.update (Widgets.svelte:34:6)
    at init (chunk-WAIAKUYY.js?v=a71937be:1719:6)
    at new Widgets (Widgets.svelte:36:60)
    at createProxiedComponent (svelte-hooks.js?v=a71937be:341:9)
    at new ProxyComponent (proxy.js?v=a71937be:242:7)
    at new Proxy<Widgets> (proxy.js?v=a71937be:349:11)
    at create_then_block (RecordPageContent.svelte:62:45)
    at update2 (chunk-WAIAKUYY.js?v=a71937be:1255:48)

I added a commit 192d652 to fix it.