* React UI: Support custom path prefixes

The challenge was that the path prefix can be set dynamically as a flag
on Prometheus, but the React app bundle is statically compiled in to
expect a given path prefix. By adding a placeholder value to the React
app's index.html and replacing it in Prometheus with the right path
prefix during serving, this injects Prometheus's path prefix into the
React app via a global const.

Threading the path prefix into the different React components could have
been done with React's Contexts (https://reactjs.org/docs/context.html),
but I found the consumer side of context values to be a bit cumbersome
(wrapping entire components in context consumers), so I ended up
preferring direct threading of the path prefix values to components that
needed them. Also, using contexts in tests is more verbose than just
passing in path prefix values directly.

Fixes https://github.com/prometheus/prometheus/issues/6163Signed-off-by: NJulius Volz <julius.volz@gmail.com>

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Signed-off-by: NJulius Volz <julius.volz@gmail.com>