(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/Mail.svg

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg width="16px" height="14px" viewBox="0 0 16 14" 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 / Mail</title>
+    <desc>Created with Sketch.</desc>
+    <g id="Icons-/-UI-/-Mail" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
+        <g id="mail" fill="#000000" fill-rule="nonzero">
+            <path d="M14.5,13 C14.7758576,13 15,12.7758576 15,12.5 L15,1.5 C15,1.22414237 14.7758576,1 14.5,1 L1.5,1 C1.22414237,1 1,1.22414237 1,1.5 L1,12.5 C1,12.7758576 1.22414237,13 1.5,13 L14.5,13 Z M14.5,14 L1.5,14 C0.671857625,14 0,13.3281424 0,12.5 L0,1.5 C0,0.671857625 0.671857625,8.8817842e-16 1.5,8.8817842e-16 L14.5,8.8817842e-16 C15.3281424,8.8817842e-16 16,0.671857625 16,1.5 L16,12.5 C16,13.3281424 15.3281424,14 14.5,14 Z M13.1833811,3.11302135 C13.3971035,2.93815756 13.7121149,2.9696587 13.8869786,3.1833811 C14.0618424,3.39710351 14.0303413,3.71211486 13.8166189,3.88697865 L8.3166189,8.38697865 C8.13243595,8.53767378 7.86756405,8.53767378 7.6833811,8.38697865 L2.1833811,3.88697865 C1.9696587,3.71211486 1.93815756,3.39710351 2.11302135,3.1833811 C2.28788514,2.9696587 2.60289649,2.93815756 2.8166189,3.11302135 L8,7.35396953 L13.1833811,3.11302135 Z" id="Combined-Shape"></path>
+        </g>
+    </g>
+</svg>