feat(common-ui): Add Markdown component.

[victorgcramos]

This diff adds the scope for Politeiagui's Markdown component.

⚠️ Depends on decred/pi-ui#429 ⚠️

Go to localhost:3007 for a better example.

Includes:

• Markdown Renderer
• Markdown Diff
• Markdown Editor and Commands
• Unit tests for created components and commands

Pros:

• Remove deprecated React-mde package
• Improve markdown diff for lines

[tiagoalvesdulce]

Looking pretty good. I think some stuff can be improved:

1. We can make our blockquotes look like github's. Example:

Will render:

But with our markdown editor the same input will render:

2. We can make our ordered list automatically jump to the next item, like github does:

3. Any plans to add keyboard shortcuts? Currently on politeia we can make text bold with Cmd+B and italic with Cmd+I. Would be nice to have key bindings for all the itens in the toolbar.
4. You forgot to add the ordered list to the toolbar.
5. I think we can add the task list to the toolbar as well. I think it would be pretty useful for proposal updates.

[tiagoalvesdulce]

Left a comment

[tiagoalvesdulce]

LGTM. Great job!! 👏