(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

static/ui-icons/UI/Maximize.svg

@@ -0,0 +1,6 @@
+<svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M14.5 1.5L9.5 6.5" stroke="white" stroke-miterlimit="10" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M6.5 9.5L1.5 14.5" stroke="white" stroke-miterlimit="10" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M8.5 1.5H14.5V7.5" stroke="white" stroke-miterlimit="10" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M1.5 8.5V14.5H7.5" stroke="white" stroke-miterlimit="10" stroke-linecap="round" stroke-linejoin="round"/>
+</svg>

static/ui-icons/UI/Minimize.svg

@@ -0,0 +1,13 @@
+<svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+<g clip-path="url(#clip0_240_7200)">
+<path d="M14.5 6.5H9.5V1.5" stroke="white" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M15.5 0.5L9.5 6.5" stroke="white" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M6.5 14.5V9.5H1.5" stroke="white" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M0.5 15.5L6.5 9.5" stroke="white" stroke-linecap="round" stroke-linejoin="round"/>
+</g>
+<defs>
+<clipPath id="clip0_240_7200">
+<rect width="16" height="16" fill="white"/>
+</clipPath>
+</defs>
+</svg>