Add Notion import

[clarani]

⚠️ THIS PR IS STILL A DRAFT ⚠️

🌟 Contributors: @emersion @Tguisnet @Castavo @NicolasRitouet @clarani
💻 Project of the Hack Days Hackathon

Purpose

The goal of this feature is to allow users to import notion workspace into Docs in 1 clic.

The user can connect to Notion by clicking on a button, select the pages he/she wants to import and the pages are directly imported into Docs.
A progress bar displays the import progression.

A lot of Notion block types are handled:

• column
• heading
• text and text formatting
• checkbox
• bullet and numbered list
• child page
• callout
• images

The unsupported blocks are replaced by a textual warning. The database pages from Notion are ignored since Docs does not handle databases for the moment.

For later

• The import is quite long since the Notion API is slow. An improvement would be to make the import run in the background.
• We don't not handle the import errors for the moment.

[emersion]

Ref #1049 which adds custom blocks support to y-provider.

[gitguardian]

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[emersion]

Note, this depends on #701 for nested documents.