(core) Forms Improvements

Summary:
 - Forms now have a reset button.
 - Choice and Reference fields in forms now have an improved select menu.
 - Formula and attachments column types are no longer mappable or visible in forms.
 - Fields in a form widget are now removed if their column is deleted.
 - The preview button in a published form widget has been replaced with a view button. It now opens the published form in a new tab.
 - A new share menu for published form widgets, with options to copy a link or embed code.
 - Forms can now have multiple sections.
 - Form widgets now indicate when publishing is unavailable (e.g. in forks or unsaved documents).
 - General improvements to form styling.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D4203

app/client/models/entities/ColumnRec.ts

@@ -68,6 +68,7 @@ export interface ColumnRec extends IRowModel<"_grist_Tables_column"> {
   disableEditData: ko.Computed<boolean>;      // True to disable editing of the data in this column.
 
   isHiddenCol: ko.Computed<boolean>;
+  isFormCol: ko.Computed<boolean>;
 
   // Returns the rowModel for the referenced table, or null, if is not a reference column.
   refTable: ko.Computed<TableRec|null>;
@@ -144,6 +145,11 @@ export function createColumnRec(this: ColumnRec, docModel: DocModel): void {
   this.disableEditData = ko.pureComputed(() => Boolean(this.summarySourceCol()));
 
   this.isHiddenCol = ko.pureComputed(() => gristTypes.isHiddenCol(this.colId()));
+  this.isFormCol = ko.pureComputed(() => (
+    !this.isHiddenCol() &&
+    this.pureType() !== 'Attachments' &&
+    !this.isRealFormula()
+  ));
 
   // Returns the rowModel for the referenced table, or null, if this is not a reference column.
   this.refTable = ko.pureComputed(() => {