Improve admin CP appearance page, allow logo upload

[tobyzerner]

We want to allow uploading a custom favicon and header image. Additionally it'd be cool to show a live preview (using an SVG) of what the color scheme will look like. Something like this:

What needs to be done:

• Add API endpoints for uploading a favicon and a header image.
• Redesign/add fields to the AppearancePage component.
• Design a simple SVG showing Flarum's basic layout, and give the appropriate elements class names so they can have their colors changed via JavaScript.
• Hook up the color buttons in the AppearancePage to update the preview SVG.

[franzliedke]

@tobscure Do you have some kind of backlog to store ideas like the SVG preview that may be worth revisiting after the first final?