(core) Initial data tables page

Summary:
- Added a new special page for viewing raw data widgets:
  - Implemented in DataTables.ts
  - Accessible only via the special URL path `/p/data`
  - Future diffs should make this page prettier and easily accessible
  - Shows a list of user tables
  - Clicking on a table name shows its `rawViewSection` by setting `GristDoc.viewModel.activeSectionId`. Note that in this case `GristDoc.viewModel` is an empty record, so this is a bit of a hack, but it works well and causes no known issues.
- Added `ViewSectionRec.isRaw` to know if the record represents a raw data widget.
- Added various restrictions in the UI for raw data widgets:
  - 'Delete widget' is disabled in the 3-dot widget menu.
  - Prevent hiding columns:
    - "Hide column" in the column context menu is disabled
    - The "VISIBLE/HIDDEN COLUMNS" section of the right panel > Table > Widget is hidden
  - The toggle bar isn't configurable to ensure that users know when raw data is filtered:
    - The filter bar always shows if and only if some filters are present
    - "Toggle Filter Bar" is hidden in:
      - Right panel > Table > Sort & Filter
      - The sort/filter menu next to the three-dot menu for widgets.
  - Other restrictions in the right panel:
    - In the Column tab:
      - 'Use separate settings' is disabled
    - In the Table tab:
      - In the Widget subtab:
        - 'Change Widget' is hidden
      - In the Data subtab:
        - 'Edit Data Selection' is hidden
        - 'SELECT BY' is hidden

Test Plan: Tested manually. The behaviour of raw data widgets may still change and they aren't easily visible to users yet.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3248

app/client/components/GridView.js

@@ -1495,6 +1495,7 @@ GridView.prototype._getColumnMenuOptions = function(copySelection) {
     numFrozen: this.viewSection.numFrozen.peek(),
     disableModify: calcFieldsCondition(copySelection.fields, f => f.disableModify.peek()),
     isReadonly: this.gristDoc.isReadonly.get() || this.isPreview,
+    isRaw: this.viewSection.isRaw(),
     isFiltered: this.isFiltered(),
     isFormula: calcFieldsCondition(copySelection.fields, f => f.column.peek().isRealFormula.peek()),
   };