(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/widgets/Switch.css

@@ -0,0 +1,48 @@
+.widget_switch {
+  position: relative;
+  margin: -1px auto;
+  width: 30px;
+  height: 17px;
+}
+
+.switch_slider {
+  position: absolute;
+  cursor: pointer;
+  top: 0;
+  left: 0;
+  right: 0;
+  bottom: 0;
+  background-color: #ccc;
+  border-radius: 17px;
+}
+
+.switch_slider:hover {
+  box-shadow: 0 0 1px #2196F3;
+}
+
+.switch_circle {
+  position: absolute;
+  cursor: pointer;
+  content: "";
+  height: 13px;
+  width: 13px;
+  left: 2px;
+  bottom: 2px;
+  background-color: white;
+  border-radius: 17px;
+}
+
+.switch_on > .switch_slider {
+  background-color: #2CB0AF;
+}
+
+.switch_on > .switch_circle {
+  -webkit-transform: translateX(13px);
+  -ms-transform: translateX(13px);
+  transform: translateX(13px);
+}
+
+.switch_transition > .switch_slider, .switch_transition > .switch_circle {
+  -webkit-transition: .4s;
+  transition: .4s;
+}