changed proxy fix to use BrowserRouter basename

[shumway743]

What this PR does / why we need it:
This PR simplifies the way the feast UI can run on a sub path, which is often needed when running behind a proxy. Instead of manually attaching an environment variable in front of each link/path, links are updated using the basename prop of BrowserRouter and exposes it as a prop on the FeastUI component. The primary benefit is maintainability (no need for links to be updated manually).

With this, a subpath can be specified by passing the basename prop, and by updating the projectListPromise on the FeastUI component in an index.js/index.tsx file.

An example running under the subpath "/feast" would look something like this:

<FeastUI basename="/feast" reactQueryClient={queryClient} feastUIConfigs={{ tabsRegistry: tabsRegistry, projectListPromise: fetch("/feast/projects-list.json", { headers: { "Content-Type": "application/json", }, }).then((res) => { return res.json(); }) }} />
Which issue(s) this PR fixes:

Fixes #

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This pull-request has been approved by: shumway743
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