scope plugins to support adjusting app base path

[digitalsadhu]

This PR makes it possible to set an app.base config value and have the whole app mounted in the server correctly based on this value. If the value is not set, the app uses the app.name as the pathname instead.

Behaviour is as follows:

• no app.base set, app base pathname set to /<app.name>
• app.base set, app base pathname set to /<app.base> n.b. app.base can be set to / to have the app mounted at the root.
• all routes are mounted under the pathname, this goes for manifest, content, fallback and any additional api routes and locally served static assets

Assets

There is an assets.base value which has some interaction with app.base. assets.base behaviour is as follows

• defaults to /static locally which is where the dist folder is served.
• If assets.base is set to an absolute url then local assets will not be served and assets will be loaded from this absolute url.
• If assets.base is not an absolute value, it will be concatenated with the app pathname (as above, either from app.base or app.name)

Examples:

// config/common.json
{
  "app": {
    "base": "/foo"
  },
  "assets": {
    "base": "/bar"
  }
}

// app served at http://localhost:8080/foo
// app assets served at http://localhost:8080/foo/bar

// config/common.json
{
  "app": {
    "base": "/foo"
  }
}

// app served at http://localhost:8080/foo
// app assets served at http://localhost:8080/foo/static

// config/common.json
{
  "app": {
    "base": "/foo"
  },
  "assets": {
    "base": "https://assets.foo.com/static"
  }
}

// app served at http://localhost:8080/foo
// app assets served at https://assets.foo.com/static

// package.json
{
  "name": "foo"
}

// config/common.json
{}

// app served at http://localhost:8080/foo
// app assets served at http://localhost:8080/foo/static

// package.json
{
  "name": "foo"
}

// config/common.json
{
  "assets": {
    "base": "/bar"
  }
}

// app served at http://localhost:8080/foo
// app assets served at http://localhost:8080/foo/bar