(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/links.ts

@@ -0,0 +1,18 @@
+/**
+ * Styling for a simple green <A HREF> link.
+ */
+
+import { colors } from 'app/client/ui2018/cssVars';
+import { styled } from 'grainjs';
+
+// Match the font-weight of buttons.
+export const cssLink = styled('a', `
+  color: ${colors.lightGreen};
+  --icon-color: ${colors.lightGreen};
+  text-decoration: none;
+  &:hover, &:focus {
+    color: ${colors.lightGreen};
+    --icon-color: ${colors.lightGreen};
+    text-decoration: underline;
+  }
+`);