Add 'with' functionality

[homostellaris]

In first iteration Want to be able to edit:

• Background color
• Background effect
• Image
• Name

Second iteration:

• People emojis
• Other copy

[homostellaris]

Can't use the 'theme' store for this because these pages will be server rendered which just does the initial render so the theme store talking 'back up the tree' won't occur until the client is hydrated and subsequent render cycles have executed which will result in a 'flash' of the base style after page load before the custom styles are shown.

Instead we should use a layout load function to pass the props to the page and just have them style things directly. We should also make sure it blocks rendering until it's complete (will this mean just the server version?).

To support we'll have to refactor to avoid the need to 'talk up the tree' to set the background color. It will have to be specified in child layouts rather than the base layout which will incur the small cost of specifying it in every top level route.