SOLID back-end

[raucao]

Adding this as a tracking issue and for general dev discussion/questions.

• https://github.com/solid-contrib/data-modules/issues/71
• https://github.com/solid-contrib/data-modules/blob/main/admin/milestones.md#f2-solid-backend-for-remotestoragejs-4k
• https://community.remotestorage.io/t/adding-solid-as-a-backend/828