[server] Allow specifying a basePath for a studio

[rexxars]

When building a studio for use outside of development and sanity.studio-context, you might want not want to host it at the root of a domain name. In order to allow for this, we need to:

• Make the CLI tool build an index file containing paths to static files that is not always fixed to /static
• Make the studio aware of the base path, prefixing the routes with it
• Make it possible to customize the base path through configuration

This PR implements the above, and also (for consistency) makes the development server use the same base path. I felt this would make it easier to debug routing issues, among other things.

Using it is as simple as adding a basePath property under the project object in sanity.json:

{
  // ...
  "project": {
    "name": "Movies studio",
    "basePath": "/studio"
  },
  // ...
}

When you access / in a development environment and a base path is set, we need to redirect to the correct location. This is currently handled inside of @sanity/default-layout since it contains the root router, but we could consider moving this to the SanityRoot component in @sanity/base in order to ensure this is "always" applied regardless of layout. Thoughts?

Fixes #692

[bjoerge]

LGTM!

[dan-dr]

Hey I found out it'd be very useful if I could specify the basePath as an option in the build command, because it'll make it much easier to use in CI and build systems (my basePath is different in production/staging).

I found how to do it, will probably PR soon.

[dan-dr]

Regarding adding the ability to specify basePath as an option in the cli, I was thinking of using it like this:

    const doc = await getDocumentElement(
      {...compilationConfig, hashes: chunkMap},
      {
        basePath: flags.basePath || (project && project.basePath) || '',
        scripts: ['vendor.bundle.js', 'app.bundle.js'].map(asset => {
          const assetPath = absoluteMatch.test(asset) ? asset : `js/${asset}`
          return {
            path: assetPath,
            hash: chunkMap[assetPath] || chunkMap[asset]
          }
        })
      }
    )

where somewhere before you do const project = config.get('project')

[dan-dr]

maybe though it's better to have it in an environment variable?