(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/UI/Dropdown.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 / UI / Dropdown</title>
+    <desc>Created with Sketch.</desc>
+    <g id="Icons-/-UI-/-Dropdown" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
+        <path d="M8,9.17463052 L10.9218273,6.1890232 C11.1684742,5.93699227 11.5683679,5.93699227 11.8150148,6.1890232 C12.0616617,6.44105413 12.0616617,6.84967701 11.8150148,7.10170794 L8,11 L4.18498519,7.10170794 C3.93833827,6.84967701 3.93833827,6.44105413 4.18498519,6.1890232 C4.43163211,5.93699227 4.83152578,5.93699227 5.0781727,6.1890232 L8,9.17463052 Z" id="Path-6" fill="#000000" fill-rule="nonzero"></path>
+    </g>
+</svg>