Adds support for deploying at sub-path behind reverse proxy

[amorey]

This PR adds support for deploying the kubetail app at a sub-path behind a reverse proxy by making the following changes:

• Adds a "base-path" config option (default: "/")
• Deploys the frontend react app in production using a template to return html generated at at run-time
• Uses the manifest.json file generated by vite at build-time to load frontend assets properly
• Passes config variables from the backend to the frontend using a runtimeConfig object injected into the html using the template
• Adds a config object to the frontend that reads properties from runtimeConfig or from the import.meta.env used by vite in development