(core) Add initial tutorials implementation

Summary:
Documents can now be flagged as tutorials, which causes them to display
Markdown-formatted slides from a special GristDocTutorial table. Tutorial
documents are forked on open, and remember the last slide a user was on.
They can be restarted too, which prepares a new fork of the tutorial.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3813

app/common/themes/GristDark.ts

@@ -430,4 +430,9 @@ export const GristDark: ThemeColors = {
   'date-picker-range-start-end-bg-hover': '#8F8F8F',
   'date-picker-range-bg': '#57575F',
   'date-picker-range-bg-hover': '#7F7F7F',
+
+  /* Tutorials */
+  'tutorials-popup-border': '#69697D',
+  'tutorials-popup-header-fg': '#FFFFFF',
+  'tutorials-popup-box-bg': '#57575F',
 };

app/common/themes/GristLight.ts

@@ -430,4 +430,9 @@ export const GristLight: ThemeColors = {
   'date-picker-range-start-end-bg-hover': '#CFCFCF',
   'date-picker-range-bg': '#EEEEEE',
   'date-picker-range-bg-hover': '#D9D9D9',
+
+  /* Tutorials */
+  'tutorials-popup-border': '#D9D9D9',
+  'tutorials-popup-header-fg': '#FFFFFF',
+  'tutorials-popup-box-bg': '#F5F5F5',
 };