Add theme caching

[mnholtz]

Follow-up of https://github.com/pixiebrix/pixiebrix-extension/pull/4373

We've run into UX problems with loading behavior for themes. Ideally, we'd like to query themes once and cache until invalidated. See discussion here: https://github.com/pixiebrix/pixiebrix-extension/pull/4373#issuecomment-1267387229

This will probably look like a themeSlice that gets hit high in the app component hierarchy.