(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/Video.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 63.1 (92452) - https://sketch.com -->
+    <title>Icons / UI / Video</title>
+    <desc>Created with Sketch.</desc>
+    <g id="Icons-/-UI-/-Video" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
+        <path d="M14.5,1 C15.3284271,1 16,1.67157288 16,2.5 L16,2.5 L16,13.5 C16,14.3284271 15.3284271,15 14.5,15 L14.5,15 L1.5,15 C0.671572875,15 0,14.3284271 0,13.5 L0,13.5 L0,2.5 C0,1.67157288 0.671572875,1 1.5,1 L1.5,1 Z M14.5,2 L1.5,2 C1.22385763,2 1,2.22385763 1,2.5 L1,2.5 L1,13.5 C1,13.7761424 1.22385763,14 1.5,14 L1.5,14 L14.5,14 C14.7761424,14 15,13.7761424 15,13.5 L15,13.5 L15,2.5 C15,2.22385763 14.7761424,2 14.5,2 L14.5,2 Z M5,4.5 C5,4.10545486 5.43557521,3.86634764 5.76843775,4.07816926 L5.76843775,4.07816926 L11.2684377,7.57816926 C11.5771874,7.77464632 11.5771874,8.22535368 11.2684377,8.42183074 L11.2684377,8.42183074 L5.76843775,11.9218307 C5.43557521,12.1336524 5,11.8945451 5,11.5 L5,11.5 Z M6,5.41 L6,10.589 L10.069,8 L6,5.41 Z" id="Combined-Shape" fill="#000000" fill-rule="nonzero"></path>
+    </g>
+</svg>