Storing settings better

[trys]

In readiness for languages, better theming, markdown and more (referenced in #35), we need to store settings in a better way than localStorage. Given idb is in place, a new table is probably the best option.

A migration will be required for the new table, key names decided, and defaults set. Any existing settings should be migrated - that should just be theme for now. This can be run in the initial table migration.

Once created, the settings page can be hooked up to the new table.