feat: add skin preview

[wa0x6e]

Summary

Closes: https://github.com/snapshot-labs/workflow/issues/418
Closes: https://github.com/snapshot-labs/workflow/issues/416

This PR:

• adds a previewer to preview the skin update in realtime
• apply the new skin on space settings change

How to test

1. Go to your space setting > whitelabel on main site
2. Preview is disabled
3. Go to your space settings > whitelabel on your space custom domain
4. Preview is enabled
5. Edit the form (colors, theme and custom logo)
6. The preview should update live
7. Save your settings
8. It should apply your new colors on save

[bonustrack]

Looks great! Some feedback:

• Ideally user should be able try preview and change colors even if the site is not setup

• If that's too much work we should at least remove the preview if the domain is not set, currently it show an incomplete sentence:

• We should allow empty values, this is part of the default form

• Can we use native color picker with <input type="color"...> when user click on the color square?

[bonustrack]

Here is how to restyle input color https://codepen.io/leemartin/pen/ExPXRzx