(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

static/ui-icons/Fields/FieldLink.svg

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg width="16px" height="16px" viewBox="0 0 16 16" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+    <!-- Generator: Sketch 52.5 (67469) - http://www.bohemiancoding.com/sketch -->
+    <title>Icons / Fields / FieldLink</title>
+    <desc>Created with Sketch.</desc>
+    <g id="Icons-/-Fields-/-FieldLink" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
+        <path d="M12.2928932,3 L8.5,3 C8.22385763,3 8,2.77614237 8,2.5 C8,2.22385763 8.22385763,2 8.5,2 L13.5,2 C13.7761424,2 14,2.22385763 14,2.5 L14,7.5 C14,7.77614237 13.7761424,8 13.5,8 C13.2238576,8 13,7.77614237 13,7.5 L13,3.70710678 L7.85355339,8.85355339 C7.65829124,9.04881554 7.34170876,9.04881554 7.14644661,8.85355339 C6.95118446,8.65829124 6.95118446,8.34170876 7.14644661,8.14644661 L12.2928932,3 Z M11,10.5 C11,10.2238576 11.2238576,10 11.5,10 C11.7761424,10 12,10.2238576 12,10.5 L12,12.5 C12,13.3284271 11.3284271,14 10.5,14 L3.5,14 C2.67157288,14 2,13.3284271 2,12.5 L2,5.5 C2,4.67157288 2.67157288,4 3.5,4 L5.5,4 C5.77614237,4 6,4.22385763 6,4.5 C6,4.77614237 5.77614237,5 5.5,5 L3.5,5 C3.22385763,5 3,5.22385763 3,5.5 L3,12.5 C3,12.7761424 3.22385763,13 3.5,13 L10.5,13 C10.7761424,13 11,12.7761424 11,12.5 L11,10.5 Z" id="Combined-Shape" fill="#000000" fill-rule="nonzero"></path>
+    </g>
+</svg>