O3-3347: SPA module should support appropriate caching headers

[ibacher]

While the ticket here is pretty narrow, this is a ground-up re-write of this module.

This rewrite drops the following features:

• Proxying files from a remote location (there are much better ways of achieving this)
• Serving the frontend from a URL other than <contextPath>/spa (we can look at adding this back in if anyone really needs it)

It adds the following features:

• Appropriately sets cache headers so that importmap.json, routes.registry.json, and the single-page itself must be validated on every request. All other files are considered static and cached for 180 days.
• Appropriate ETags are generated for all files, allowing less data to be sent over the wire if nothing has changed content-wise.
• GZip compression is enabled for all files for user-agents that support it, again reducing the amount of traffic served over the wire.
• Support for defining a custom frontend folder using runtime or system instead of a global property.

Implementation-wise, this relies heavily on Spring MVC's machinery to actually serve requests, so the previous servlet has been replaced with a controller. Additionally, the calls to get GP properties have been shifted into a GP listener. This means that we only need to update the spa.local.directory if someone actually changes the setting.