(core) move client code to core

Summary:
This moves all client code to core, and makes minimal fix-ups to
get grist and grist-core to compile correctly.  The client works
in core, but I'm leaving clean-up around the build and bundles to
follow-up.

Test Plan: existing tests pass; server-dev bundle looks sane

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2627

app/client/ui2018/loaders.ts

@@ -0,0 +1,22 @@
+import {colors} from 'app/client/ui2018/cssVars';
+import {keyframes, styled} from 'grainjs';
+
+const rotate360 = keyframes(`
+  from { transform: rotate(45deg); }
+  75% { transform: rotate(405deg); }
+  to { transform: rotate(405deg); }
+`);
+
+/**
+ * Creates a 32x32 pixel loading spinner. Use by calling `loadingSpinner()`.
+ */
+export const loadingSpinner = styled('div', `
+  display: inline-block;
+  box-sizing: border-box;
+  width: 32px;
+  height: 32px;
+  border-radius: 32px;
+  border: 4px solid ${colors.darkGrey};
+  border-top-color: ${colors.lightGreen};
+  animation: ${rotate360} 1s ease-out infinite;
+`);