Adds basic Space theming configuration: `show_header`, `header_bg_color`, `header_txt_color`

[rosschapman]

This changeset enables Space admins to:

• [x] Enable and disable add a sitewide header that will include Space name and Section Navigation
• [x] Customize the header background color
• [x] Customize the header text color

Additional implementation details and considerations:

• The header is a common responsive design that is stacked vertically for small screens and expands horizontally for larger screens

https://github.com/zinc-collective/convene/assets/5185/7b063ee8-5571-42fd-b79f-55695b03c477

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Reference 🎟️ : https://github.com/zinc-collective/convene/issues/2266

[rosschapman]

@zinc-collective/convene-maintainers @zinc-collective/convene-contributors

Here's a spike to add basic Site Branding capabilities to Convene. I'm aiming for the simplest implementation at the start here with new static attributes on a Space vs increased sophistication like making headers and footer associative objects, a template system, etc...:

• Header is on/off, will display space-wide, is not configurable, only shows space name and section links with default styling (to come)
• Footer is customizable as action text
• Brand color only applies to header and footer

Does this feel like a reasonable place to start?

NOTE: Spike is deliberately missing db bits.

Sample:

[zspencer]

Can we get away with just the brand color and header off/on for now?