[Feature] Support deploying Appsmith on a subpath

Currently, the `index.html` page of Appsmith requests for resources starting at `/`, like `/static/main.js` etc. This makes it hard to have an Appsmith instance deployed at a non-root path in a domain. Like `http://example.com/appsmith` or something.

This will be useful in setups where people don't have access to setup an A record for a subdomain, but do have access setup multiple applications over paths by configuring their routing.

[Front conversations](https://app.frontapp.com/open/top_71wcs)