(core) move home server into core

Summary: This moves enough server material into core to run a home server.  The data engine is not yet incorporated (though in manual testing it works when ported).

Test Plan: existing tests pass

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2552

app/plugin/InternalImportSourceAPI.ts

@@ -0,0 +1,24 @@
+import { RenderTarget } from './RenderOptions';
+
+import { ImportSource } from './ImportSourceAPI';
+
+export * from  './ImportSourceAPI';
+
+/**
+ * This internal interface is implemented by grist-plugin-api.ts to support
+ * `grist.addImporter(...)`. This is this interface that grist stubs to calls
+ * `ImportSourceAPI`. However, some of the complexity (ie: rendering targets) is hidden from the
+ * plugin author which implements directly the simpler `ImportSourceAPI`.
+ *
+ * Reason for this interface is because we want to have the `inlineTarget` parameter but we don't
+ * want plugin author to have it.
+ */
+export interface InternalImportSourceAPI {
+  /**
+   * The `inlineTarget` argument which will be passed to the implementation of this method, can be
+   * used as follow `grist.api.render('index.html', inlineTarget)` to embbed `index.html` in the
+   * import panel. Or it can be ignored and use `'fullscreen'` in-place. It is used in
+   * `grist.addImporter(...)` according to the value of the `mode` argument.
+   */
+  getImportSource(inlineTarget: RenderTarget): Promise<ImportSource|undefined>;
+}