Massive rework of platform layer user/organization

The goal of this rework is to make it so I can pass around platform.User
instead of a pair of models.Organization and models.User. This is useful
for reason I kind of forget now, but it started with working on
notifications and ballooned massively from there into refactoring a
number of things that were bugging me.

This also includes a tiny amount of work on server-side events (SSE).

 * background stuff lives inside the platform now, which I need for
   having it push updates through SSE
 * userfile now lives in the platform, under file, so other platform
   functions can safely use it
 * oauth is broken into pieces and inside platform because other stuff
   was calling it already, but badly.
 * notifications go into the platform as well

platform/background.go

@@ -0,0 +1,14 @@
+package platform
+
+import (
+	"context"
+
+	"github.com/Gleipnir-Technology/nidus-sync/platform/background"
+)
+
+func BackgroundStart(ctx context.Context) {
+	background.Start(ctx)
+}
+func BackgroundWaitForExit() {
+	background.WaitForExit()
+}