Move app/src (mostly) to flipper-ui-core/src

Summary:
This diff moves all UI code from app/src to app/flipper-ui-core. That is now slightly too much (e.g. node deps are not removed yet), but from here it should be easier to move things out again, as I don't want this diff to be open for too long to avoid too much merge conflicts.

* But at least flipper-ui-core is Electron free :)
* Killed all cross module imports as well, as they where now even more in the way
* Some unit test needed some changes, most not too big (but emotion hashes got renumbered in the snapshots, feel free to ignore that)
* Found some files that were actually meaningless (tsconfig in plugins, WatchTools files, that start generating compile errors, removed those

Follow up work:
* make flipper-ui-core configurable, and wire up flipper-server-core in Electron instead of here
* remove node deps (aigoncharov)
* figure out correct place to load GKs, plugins, make intern requests etc., and move to the correct module
* clean up deps

Reviewed By: aigoncharov

Differential Revision: D32427722

fbshipit-source-id: 14fe92e1ceb15b9dcf7bece367c8ab92df927a70

desktop/flipper-ui-core/src/sandy-chrome/ContentContainer.tsx

@@ -0,0 +1,20 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+import {Layout, styled} from '../ui';
+import {theme} from 'flipper-plugin';
+
+export const ContentContainer = styled(Layout.Container)({
+  flex: 1,
+  overflow: 'hidden',
+  background: theme.backgroundDefault,
+  border: `1px solid ${theme.dividerColor}`,
+  borderRadius: theme.containerBorderRadius,
+  boxShadow: `0px 0px 5px rgba(0, 0, 0, 0.05), 0px 0px 1px rgba(0, 0, 0, 0.05)`,
+});