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/.vscode/settings.json

@@ -16,10 +16,10 @@
     }
   ],
   "[typescriptreact]": {
-    "editor.defaultFormatter": "dbaeumer.vscode-eslint"
+    "editor.defaultFormatter": "esbenp.prettier-vscode"
   },
   "[typescript]": {
-    "editor.defaultFormatter": "dbaeumer.vscode-eslint"
+    "editor.defaultFormatter": "esbenp.prettier-vscode"
   },
   "[javascript]": {
     "editor.defaultFormatter": "dbaeumer.vscode-eslint"