Closes #1866

This PR:

• adds a new layout blocks concept to the application (currently charts and text) with a potential to add more in the future,
• makes it possible to use Markdown (including GFM and sanitized HTML extensions) in chart title and description. It's also possible to use keyboard shortcuts to style the text (e.g. CMD + b for bold, CMD + I for italic, CMD + u) for underlined,
• adds text blocks to dashboard layouts (same Markdown features as above) with an auto-sizing behavior when the text is being entered,
• adds pull request template.

How to test

1. Go to this link.
2. Add a new chart.
3. Proceed to layout options.
4. Switch to a dashboard / free canvas layout.
5. See that there's an Objects section.
6. Add a new text object.
7. ✅ See that it's possible to edit the text and use Markdown syntax.
8. ✅ Add a few new lines to see that the size of the text element automatically adjusts.
9. Switch to a chart or dashboard title / description.
10. ✅ See that you can also write Markdown content there.

TODO

• Add UI buttons to control Markdown text styling.
• Fix switching between layouts after adding a new text block (refs still hold old object without the new block, which leads to its removal).
• (Another iteration) Make sure the min height of text elements takes all locales into account.
• Update CHANGELOG.
• Make sure auto-resizing behavior works in breakpoints other than xl.
• Make sure ensuring of correct layout is working as expected with text blocks.
• Make sure text blocks can be added and work as expected in layouts other than free canvas.
• Remove vertical resizing buttons from text blocks.