(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/FieldBuilder.css

@@ -0,0 +1,32 @@
+.transform_editor {
+  width: 90%;
+  margin: 5px auto;
+  border: 1px solid #DDDDDD;
+}
+
+.transform_menu {
+  padding: 5px 0;
+  margin: 5px;
+  border-top: 1px solid rgba(200, 200, 200, .3);
+  border-bottom: 1px solid rgba(200, 200, 200, .3);
+}
+
+.fieldbuilder_settings {
+  background-color: #e8e8e8;
+  margin: 1rem -1px -4px -1px;
+  padding-bottom: 1px;
+}
+
+.fieldbuilder_settings_header {
+  height: 2rem;
+  margin-top: 0px;
+  margin-bottom: 0px;
+}
+
+.fieldbuilder_settings_button {
+  display: inline-block;
+  float: right;
+  padding: 0 1rem;
+  border-radius: 5px;
+  background-color: lightgrey;
+}