Handle "site preview" when default_context is not "web" context or when manager is on its own (sub)domain

### What does it do ?

Compute the `default_context` URL, make it available in `MODx.config.default_site_url` (manager) and use it in `MODx.preview()`

### Why is it needed ?

When the `default_context` system setting is set to something other than "web", hitting Content > Site preview opens the web context.
When the manager is on its own (sub)domain, hitting Content > Site preview opens a new manager.

With this PR, both of the above cases should be fixed, and Site preview should open the URL of `default_context` (falling back to web context)