Related to https://github.com/discourse/discourse/pull/30893
As part of the theme overhauling project, we're making each theme fully
own/control its color palette which can be edited directly on the theme
page. To make this possible, we need to introduce a special type of
color palettes that are marked as "owned by a theme" in the database
which aren't displayed in the admin color palettes page and can't be
edited from it. This commit is the first step of this change; it adds a new
join table to associate a color palette with a theme. For now, we're
keeping the relationship one-to-one (hence the `UNIQUE` indexes), but we
may later change it to one-to-many.
Internal topic: t/141648.