(core) Polish forms

Summary:
  - Updates styling of form submitted page.
  - Tweaks styling of checkboxes, labels, and questions on form page.
  - Adds new form 404 page.
  - Adds checkbox to not show warning again when publishing or un-publishing a form.
  - Excludes formula, hidden, and attachment columns in submitted form data.
  - Adds placeholder text to form configuration inputs.
  - Improves dark mode styling in Form widget.
  - Updates default title and description of new forms.
  - Updates styling of Form widget buttons.
  - Fixes form success text input handling.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D4170

app/common/themes/GristDark.ts

@@ -4,6 +4,7 @@ export const GristDark: ThemeColors = {
   /* Text */
   'text': '#EFEFEF',
   'text-light': '#A4A4B1',
+  'text-medium': '#D5D5D5',
   'text-dark': '#FFFFFF',
   'text-error': '#E63946',
   'text-error-hover': '#FF5C5C',

app/common/themes/GristLight.ts

@@ -4,6 +4,7 @@ export const GristLight: ThemeColors = {
   /* Text */
   'text': '#262633',
   'text-light': '#929299',
+  'text-medium': '#494949',
   'text-dark': 'black',
   'text-error': '#D0021B',
   'text-error-hover': '#A10000',